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SubjectRe: For review: seccomp_user_notif(2) manual page
Hi Jann,

So, first off, thank you for the detailed review. I really
appreciate it! I've changed various pieces, and still have
a few questions below.

On 9/30/20 5:53 PM, Jann Horn wrote:
> On Wed, Sep 30, 2020 at 1:07 PM Michael Kerrisk (man-pages)
> <> wrote:
>> I knew it would be a big ask, but below is kind of the manual page
>> I was hoping you might write [1] for the seccomp user-space notification
>> mechanism. Since you didn't (and because 5.9 adds various new pieces
>> that also will need documenting [2]), I did :-). But of course I may
>> have made mistakes...
> [...]
>> seccomp_user_notif - Seccomp user-space notification mechanism
>> #include <linux/seccomp.h>
>> #include <linux/filter.h>
>> #include <linux/audit.h>
>> int seccomp(unsigned int operation, unsigned int flags, void *args);
> Should the ioctl() calls be listed here, similar to e.g. the SYNOPSIS
> of the ioctl_* manpages?

Yes, good idea. I added:

int ioctl(int fd, SECCOMP_IOCTL_NOTIF_RECV,
struct seccomp_notif *req);
int ioctl(int fd, SECCOMP_IOCTL_NOTIF_SEND,
struct seccomp_notif_resp *req);
int ioctl(int fd, SECCOMP_IOCTL_NOTIF_ID_VALID, __u64 *id);
>> This page describes the user-space notification mechanism pro‐
>> vided by the Secure Computing (seccomp) facility. As well as the
>> operation described in seccomp(2), this mechanism involves the
>> use of a number of related ioctl(2) operations (described below).
>> Overview
>> In conventional usage of a seccomp filter, the decision about how
>> to treat a particular system call is made by the filter itself.
>> The user-space notification mechanism allows the handling of the
>> system call to instead be handed off to a user-space process.
>> The advantages of doing this are that, by contrast with the sec‐
>> comp filter, which is running on a virtual machine inside the
>> kernel, the user-space process has access to information that is
>> unavailable to the seccomp filter and it can perform actions that
>> can't be performed from the seccomp filter.
>> In the discussion that follows, the process that has installed
>> the seccomp filter is referred to as the target, and the process
> Technically, this definition of "target" is a bit inaccurate because:
> - seccomp filters are inherited
> - seccomp filters apply to threads, not processes
> - seccomp filters can be semi-remotely installed via TSYNC

(Nice summary.)

> (I assume that in manpages, we should try to go for the "a task is a
> thread and a thread group is a process" definition, right?)


> Perhaps "the threads on which the seccomp filter is installed are
> referred to as the target", or something like that would be better?

Thanks. It's always hugely helpful to get a suggested wording, even
if I still feel the need to rework it (which I don't in this case).
The sentence now reads:

In the discussion that follows, the thread(s) on which the seccomp
filter is installed are referred to as the target, and the process
that is notified by the user-space notification mechanism is
referred to as the supervisor.

>> that is notified by the user-space notification mechanism is
>> referred to as the supervisor. An overview of the steps per‐
>> formed by these two processes is as follows:
>> 1. The target process establishes a seccomp filter in the usual
>> manner, but with two differences:
>> · The seccomp(2) flags argument includes the flag SECCOMP_FIL‐
>> TER_FLAG_NEW_LISTENER. Consequently, the return value of
>> the (successful) seccomp(2) call is a new "listening" file
>> descriptor that can be used to receive notifications.
>> · In cases where it is appropriate, the seccomp filter returns
>> the action value SECCOMP_RET_USER_NOTIF. This return value
>> will trigger a notification event.
>> 2. In order that the supervisor process can obtain notifications
>> using the listening file descriptor, (a duplicate of) that
>> file descriptor must be passed from the target process to the
>> supervisor process. One way in which this could be done is by
>> passing the file descriptor over a UNIX domain socket connec‐
>> tion between the two processes (using the SCM_RIGHTS ancillary
>> message type described in unix(7)). Another possibility is
>> that the supervisor might inherit the file descriptor via
>> fork(2).
> With the caveat that if the supervisor inherits the file descriptor
> via fork(), that (more or less) implies that the supervisor is subject
> to the same filter (although it could bypass the filter using a helper
> thread that responds SECCOMP_USER_NOTIF_FLAG_CONTINUE, but I don't
> expect any clean software to do that).

It's a good thing no one ever writes unclean software...

Thanks for catching this; Tycho did also. It was a thinko on my part
to forget that if one used fork(), the supervisor would inherit the
filter. I've simply removed the sentence mentioning fork().

>> 3. The supervisor process will receive notification events on the
>> listening file descriptor. These events are returned as
>> structures of type seccomp_notif. Because this structure and
>> its size may evolve over kernel versions, the supervisor must
>> first determine the size of this structure using the sec‐
>> comp(2) SECCOMP_GET_NOTIF_SIZES operation, which returns a
>> structure of type seccomp_notif_sizes. The supervisor allo‐
>> cates a buffer of size seccomp_notif_sizes.seccomp_notif bytes
>> to receive notification events. In addition,the supervisor
>> allocates another buffer of size seccomp_notif_sizes.sec‐
>> comp_notif_resp bytes for the response (a struct sec‐
>> comp_notif_resp structure) that it will provide to the kernel
>> (and thus the target process).
>> 4. The target process then performs its workload, which includes
>> system calls that will be controlled by the seccomp filter.
>> Whenever one of these system calls causes the filter to return
>> the SECCOMP_RET_USER_NOTIF action value, the kernel does not
>> execute the system call; instead, execution of the target
>> process is temporarily blocked inside the kernel and a notifi‐
> where "blocked" refers to the interruptible, restartable kind - if the
> child receives a signal with an SA_RESTART signal handler in the
> meantime, it'll leave the syscall, go through the signal handler, then
> restart the syscall again and send the same request to the supervisor
> again. so the supervisor may see duplicate syscalls.

So, I partially demonstrated what you describe here, for two example
system calls (epoll_wait() and pause()). But I could not exactly
demonstrate things as I understand you to be describing them. (So,
I'm not sure whether I have not understood you correctly, or
if things are not exactly as you describe them.)

Here's a scenario (A) that I tested:

1. Target installs seccomp filters for a blocking syscall
(epoll_wait() or pause(), both of which should never restart,
regardless of SA_RESTART)
2. Target installs SIGINT handler with SA_RESTART
3. Supervisor is sleeping (i.e., is not blocked in
4. Target makes a blocking system call (epoll_wait() or pause()).
5. SIGINT gets delivered to target; handler gets called;
***and syscall gets restarted by the kernel***

That last should never happen, of course, and is a result of the
combination of both the user-notify filter and the SA_RESTART flag.
If one or other is not present, then the system call is not

So, as you note below, the UAPI gets broken a little.

However, from your description above I had understood that
something like the following scenario (B) could occur:

1. Target installs seccomp filters for a blocking syscall
(epoll_wait() or pause(), both of which should never restart,
regardless of SA_RESTART)
2. Target installs SIGINT handler with SA_RESTART
3. Supervisor performs SECCOMP_IOCTL_NOTIF_RECV operation (which
4. Target makes a blocking system call (epoll_wait() or pause()).
5. Supervisor gets seccomp user-space notification (i.e.,
6. SIGINT gets delivered to target; handler gets called;
and syscall gets restarted by the kernel
7. Supervisor performs another SECCOMP_IOCTL_NOTIF_RECV operation
which gets another notification for the restarted system call.

However, I don't observe such behavior. In step 6, the syscall
does not get restarted by the kernel, but instead returns -1/EINTR.
Perhaps I have misconstructed my experiment in the second case, or
perhaps I've misunderstood what you meant, or is it possibly the
case that things are not quite as you said?

> What's really gross here is that signal(7) promises that some syscalls
> like epoll_wait(2) never restart, but seccomp doesn't know about that;
> if userspace installs a filter that uses SECCOMP_RET_USER_NOTIF for a
> non-restartable syscall, the result is that UAPI gets broken a little
> bit. Luckily normal users of seccomp probably won't use
> SECCOMP_RET_USER_NOTIF for restartable syscalls, but if someone does
> want to do that, we might have to add some "suppress syscall
> restarting" flag into the seccomp action value, or something like
> that... yuck.

Yes, the UAPI breakage is a bit sad (although, likely to be rarely
encountered, as you note). I'm inclined to add a note about this in
in BUGS, but beforehand I'm interested in hearing your thoughts on
scenario B above.

>> cation event is generated on the listening file descriptor.
>> 5. The supervisor process can now repeatedly monitor the listen‐
>> ing file descriptor for SECCOMP_RET_USER_NOTIF-triggered
>> events. To do this, the supervisor uses the SEC‐
>> COMP_IOCTL_NOTIF_RECV ioctl(2) operation to read information
>> about a notification event; this operation blocks until an
> (interruptably - but I guess that maybe doesn't have to be said
> explicitly here?)

Yes, I think so. The general assumption is that syscalls block
interruptibly, unless text in a manual page that says
"uninterruptible". (Postscript: Christian made a similar comment,
so I decided to explicitly note that it's an interruptible sleep.)

>> event is available.
> Maybe we should note here that you can use the multi-fd-polling APIs
> (select/poll/epoll) instead, and that if the notification goes away
> before you call SECCOMP_IOCTL_NOTIF_RECV, the ioctl will return
> -ENOENT instead of blocking, and therefore as long as nobody else
> reads from the same fd, you can assume that after the fd reports as
> readable, you can call SECCOMP_IOCTL_NOTIF_RECV once without blocking.

I'd rather not add this info in the overview section, which is
already longer than I would like. But I did add some details

The file descriptor returned when seccomp(2) is employed with the
SECCOMP_FILTER_FLAG_NEW_LISTENER flag can be monitored using
poll(2), epoll(7), and select(2). When a notification is pending,
these interfaces indicate that the file descriptor is readable.
Following such an indication, a subsequent SEC‐
COMP_IOCTL_NOTIF_RECV ioctl(2) will not block, returning either
information about a notification or else failing with the error
EINTR if the target process has been killed by a signal or its
system call has been interrupted by a signal handler.


> Exceeeeept that this part looks broken:
> if (mutex_lock_interruptible(&filter->notify_lock) < 0)
> return EPOLLERR;
> which I think means that we can have a race where a signal arrives
> while poll() is trying to add itself to the waitqueue of the seccomp
> fd, and then we'll get a spurious error condition reported on the fd.
> That's a kernel bug, I'd say.

Sigh... Writing documentation helps find bugs. Who knew?

>> The operation returns a seccomp_notif
>> structure containing information about the system call that is
>> being attempted by the target process.
>> 6. The seccomp_notif structure returned by the SEC‐
>> COMP_IOCTL_NOTIF_RECV operation includes the same information
>> (a seccomp_data structure) that was passed to the seccomp fil‐
>> ter. This information allows the supervisor to discover the
>> system call number and the arguments for the target process's
>> system call. In addition, the notification event contains the
>> PID of the target process.
> That's a PIDTYPE_PID, which the manpages call a "thread ID".

Yes. Fixed now. More generally, I've swept through the page replacing
various instances of "target process" with either "target thread", or
often just "target".

>> The information in the notification can be used to discover
>> the values of pointer arguments for the target process's sys‐
>> tem call. (This is something that can't be done from within a
>> seccomp filter.) To do this (and assuming it has suitable
>> permissions), the supervisor opens the corresponding
>> /proc/[pid]/mem file,
> ... which means that here we might have to get into the weeds of how
> actually /proc has invisible directories for every TID, even though
> only the ones for PIDs are visible, and therefore you can just open
> /proc/[tid]/mem and it'll work fine?

I myself was unaware of this for years until I *accidentally* made use
of the feature in one of my test programs and then a while later got to
asking myself "how come that worked?".

About two years ago, I added some text (@) to explain this in proc(5)
near the start of the page:

Underneath /proc, there are the following general groups of files
and subdirectories:

/proc/[pid] subdirectories
Underneath each of the /proc/[pid] directories, a task sub‐
directory contains subdirectories of the form task/[tid],

The /proc/[pid] subdirectories are visible when iterating
through /proc with getdents(2) (and thus are visible when
one uses ls(1) to view the contents of /proc).

/proc/[tid] subdirectories
@ Each one of these subdirectories contains files and subdi‐
@ rectories exposing information about the thread with the
@ corresponding thread ID. The contents of these directories
@ are the same as the corresponding /proc/[pid]/task/[tid]
@ directories.

@ The /proc/[tid] subdirectories are not visible when iterat‐
@ ing through /proc with getdents(2) (and thus are not visi‐
@ ble when one uses ls(1) to view the contents of /proc).

I think I'll just drop a cross reference to proc(5) into the text in

>> seeks to the memory location that corre‐
>> sponds to one of the pointer arguments whose value is supplied
>> in the notification event, and reads bytes from that location.
>> (The supervisor must be careful to avoid a race condition that
>> can occur when doing this; see the description of the SEC‐
>> COMP_IOCTL_NOTIF_ID_VALID ioctl(2) operation below.) In addi‐
>> tion, the supervisor can access other system information that
>> is visible in user space but which is not accessible from a
>> seccomp filter.
>> ┌─────────────────────────────────────────────────────┐
>> │FIXME │
>> ├─────────────────────────────────────────────────────┤
>> │Suppose we are reading a pathname from /proc/PID/mem │
>> │for a system call such as mkdir(). The pathname can │
>> │be an arbitrary length. How do we know how much (how │
>> │many pages) to read from /proc/PID/mem? │
>> └─────────────────────────────────────────────────────┘
> It can't be an arbitrary length. While pathnames *returned* from the
> kernel in some places can have different limits, strings supplied as
> path arguments *to* the kernel AFAIK always have an upper limit of
> PATH_MAX, else you get -ENAMETOOLONG. See getname_flags().

Yes, another thinko on my part. I removed this FIXME.

>> 7. Having obtained information as per the previous step, the
>> supervisor may then choose to perform an action in response to
>> the target process's system call (which, as noted above, is
>> not executed when the seccomp filter returns the SEC‐
>> COMP_RET_USER_NOTIF action value).

As you probably saw, I give SECCOMP_USER_NOTIF_FLAG_CONTINUE a brief
mention a couple of paragraphs later, and then go into rather more
detail later in the page. (Or do you still think something needs

>> One example use case here relates to containers. The target
>> process may be located inside a container where it does not
>> have sufficient capabilities to mount a filesystem in the con‐
>> tainer's mount namespace. However, the supervisor may be a
>> more privileged process that that does have sufficient capa‐
> nit: s/that that/that/

Thanks. Fixed.

>> bilities to perform the mount operation.
>> 8. The supervisor then sends a response to the notification. The
>> information in this response is used by the kernel to con‐
>> struct a return value for the target process's system call and
>> provide a value that will be assigned to the errno variable of
>> the target process.
>> The response is sent using the SECCOMP_IOCTL_NOTIF_RECV
>> ioctl(2) operation, which is used to transmit a sec‐
>> comp_notif_resp structure to the kernel. This structure
>> includes a cookie value that the supervisor obtained in the
>> seccomp_notif structure returned by the SEC‐
>> COMP_IOCTL_NOTIF_RECV operation. This cookie value allows the
>> kernel to associate the response with the target process.
> (unless if the target thread entered a signal handler or was killed in
> the meantime)

Yes, but I think I have this adequately covered in the errors described
later in the page for SECCOMP_IOCTL_NOTIF_RECV. (I have now added the
target-process-terminated case to the orror text.)

ENOENT The blocked system call in the target has been
interrupted by a signal handler or the target
process has terminated.

Is that sufficient?

>> 9. Once the notification has been sent, the system call in the
>> target process unblocks, returning the information that was
>> provided by the supervisor in the notification response.
>> As a variation on the last two steps, the supervisor can send a
>> response that tells the kernel that it should execute the target
>> process's system call; see the discussion of SEC‐
>> ioctl(2) operations
>> The following ioctl(2) operations are provided to support seccomp
>> user-space notification. For each of these operations, the first
>> (file descriptor) argument of ioctl(2) is the listening file
>> descriptor returned by a call to seccomp(2) with the SECCOMP_FIL‐
>> This operation is used to obtain a user-space notification
>> event. If no such event is currently pending, the opera‐
>> tion blocks until an event occurs.
> Not necessarily; for every time a process entered a signal handler or
> was killed while a notification was pending, a call to

Yes, but do you not consider this sufficiently covered by the
(updated) error text that appears later? (See below.)

>> The third ioctl(2)
>> argument is a pointer to a structure of the following form
>> which contains information about the event. This struc‐
>> ture must be zeroed out before the call.
>> struct seccomp_notif {
>> __u64 id; /* Cookie */
>> __u32 pid; /* PID of target process */
> (TID, not PID)

Thanks. Fixed.

>> __u32 flags; /* Currently unused (0) */
>> struct seccomp_data data; /* See seccomp(2) */
>> };
>> The fields in this structure are as follows:
>> id This is a cookie for the notification. Each such
>> cookie is guaranteed to be unique for the corre‐
>> sponding seccomp filter. In other words, this
>> cookie is unique for each notification event from
>> the target process.
> That sentence about "target process" looks wrong to me. The cookies
> are unique across notifications from the filter, but there can be
> multiple filters per thread, and multiple threads per filter.

Thanks. I simply removed that last sentence.

>> The cookie value has the fol‐
>> lowing uses:
>> · It can be used with the SEC‐
>> COMP_IOCTL_NOTIF_ID_VALID ioctl(2) operation to
>> verify that the target process is still alive.
>> · When returning a notification response to the
>> kernel, the supervisor must include the cookie
>> value in the seccomp_notif_resp structure that is
>> specified as the argument of the SEC‐
>> pid This is the PID of the target process that trig‐
>> gered the notification event.
>> ┌─────────────────────────────────────────────────────┐
>> │FIXME │
>> ├─────────────────────────────────────────────────────┤
>> │This is a thread ID, rather than a PID, right? │
>> └─────────────────────────────────────────────────────┘
> Yeah.

Thanks. I've made various fixes.

>> flags This is a bit mask of flags providing further
>> information on the event. In the current implemen‐
>> tation, this field is always zero.
>> data This is a seccomp_data structure containing infor‐
>> mation about the system call that triggered the
>> notification. This is the same structure that is
>> passed to the seccomp filter. See seccomp(2) for
>> details of this structure.
>> On success, this operation returns 0; on failure, -1 is
>> returned, and errno is set to indicate the cause of the
>> error. This operation can fail with the following errors:
>> EINVAL (since Linux 5.5)
>> The seccomp_notif structure that was passed to the
>> call contained nonzero fields.
>> ENOENT The target process was killed by a signal as the
>> notification information was being generated.
> Not just killed, interruption with a signal handler has the same effect.

Ah yes! Thanks. I added that as well.

ENOENT The target thread was killed by a signal as the
notification information was being generated, or the
target's (blocked) system call was interrupted by a
signal handler.


>> ┌─────────────────────────────────────────────────────┐
>> │FIXME │
>> ├─────────────────────────────────────────────────────┤
>> │From my experiments, it appears that if a SEC‐ │
>> │COMP_IOCTL_NOTIF_RECV is done after the target │
>> │process terminates, then the ioctl() simply blocks │
>> │(rather than returning an error to indicate that the │
>> │target process no longer exists). │
>> │ │
>> │I found that surprising, and it required some con‐ │
>> │tortions in the example program. It was not possi‐ │
>> │ble to code my SIGCHLD handler (which reaps the zom‐ │
>> │bie when the worker/target process terminates) to │
>> │simply set a flag checked in the main handleNotifi‐ │
>> │cations() loop, since this created an unavoidable │
>> │race where the child might terminate just after I │
>> │had checked the flag, but before I blocked (for‐ │
>> │ever!) in the SECCOMP_IOCTL_NOTIF_RECV operation. │
>> │Instead, I had to code the signal handler to simply │
>> │call _exit(2) in order to terminate the parent │
>> │process (the supervisor). │
>> │ │
>> │Is this expected behavior? It seems to me rather │
>> │desirable that SECCOMP_IOCTL_NOTIF_RECV should give │
>> │an error if the target process has terminated. │
>> └─────────────────────────────────────────────────────┘
> You could poll() the fd first. But yeah, it'd probably be a good idea
> to change that.

Ah! It was only after reading some comments from Christian that I
realized how poll() works here. I'll make some additions to the
page about the poll() details. (See my reply to Christian that should
land at about the same time as this mail.)

> [...]
>> In the above scenario, the risk is that the supervisor may
>> try to access the memory of a process other than the tar‐
>> get. This race can be avoided by following the call to
>> open with a SECCOMP_IOCTL_NOTIF_ID_VALID operation to ver‐
>> ify that the process that generated the notification is
>> still alive. (Note that if the target process subse‐
>> quently terminates, its PID won't be reused because there
> That's wrong, the PID can be reused, but the /proc/$pid directory is
> internally not associated with the numeric PID, but, conceptually
> speaking, with a specific incarnation of the PID, or something like
> that. (Actually, it is associated with the "struct pid", which is not
> reused, instead of the numeric PID.)

Thanks. I simplified the last sentence of the paragraph:

In the above scenario, the risk is that the supervisor may
try to access the memory of a process other than the tar‐
get. This race can be avoided by following the call to
open(2) with a SECCOMP_IOCTL_NOTIF_ID_VALID operation to
verify that the process that generated the notification is
still alive. (Note that if the target terminates after the
latter step, a subsequent read(2) from the file descriptor
will return 0, indicating end of file.)

I think that's probably enough detail.

>> remains an open reference to the /proc[pid]/mem file; in
>> this case, a subsequent read(2) from the file will return
>> 0, indicating end of file.)
>> On success (i.e., the notification ID is still valid),
>> this operation returns 0 On failure (i.e., the notifica‐
> nit: s/returns 0/returns 0./

Thanks. Fixed.

>> tion ID is no longer valid), -1 is returned, and errno is
>> set to ENOENT.
> [...]
>> Two kinds of response are possible:
>> · A response to the kernel telling it to execute the tar‐
>> get process's system call. In this case, the flags
>> field includes SECCOMP_USER_NOTIF_FLAG_CONTINUE and the
>> error and val fields must be zero.
>> This kind of response can be useful in cases where the
>> supervisor needs to do deeper analysis of the target's
>> system call than is possible from a seccomp filter
>> (e.g., examining the values of pointer arguments), and,
>> having verified that the system call is acceptable, the
>> supervisor wants to allow it to proceed.
> "allow" sounds as if this is an access control thing, but this
> mechanism should usually not be used for access control (unless the
> "seccomp" syscall is blocked).

Yes, Kees has also raised this point.

> Maybe reword as "having decided that
> the system call does not require emulation by the supervisor, the
> supervisor wants it to execute normally", or something like that?

Great! More suggested wordings! Thank you :-).

I tweaked slightly:

... having decided that the system call does not require emulation
by the supervisor, the supervisor wants the system call to
be executed normally in the target.

> [...]
>> On success, this operation returns 0; on failure, -1 is
>> returned, and errno is set to indicate the cause of the
>> error. This operation can fail with the following errors:
>> A response to this notification has already been
>> sent.
>> EINVAL An invalid value was specified in the flags field.
>> EINVAL The flags field contained SEC‐
>> COMP_USER_NOTIF_FLAG_CONTINUE, and the error or val
>> field was not zero.
>> ENOENT The blocked system call in the target process has
>> been interrupted by a signal handler.
> (you could also get this if a response has already been sent, instead
> of EINPROGRESS - the only difference is whether the target thread has
> picked up the response yet)

Got it. I don't think I'll try to work that detail into the page
(unless you really think I should, but since you made this a
parenthetical comment, perhaps you don't think it's necessary).

>> The file descriptor returned when seccomp(2) is employed with the
>> SECCOMP_FILTER_FLAG_NEW_LISTENER flag can be monitored using
>> poll(2), epoll(7), and select(2). When a notification is pend‐
>> ing, these interfaces indicate that the file descriptor is read‐
>> able.
> We should probably also point out somewhere that, as
> include/uapi/linux/seccomp.h says:
> * Similar precautions should be applied when stacking SECCOMP_RET_USER_NOTIF
> * or SECCOMP_RET_TRACE. For SECCOMP_RET_USER_NOTIF filters acting on the
> * same syscall, the most recently added filter takes precedence. This means
> * that the new SECCOMP_RET_USER_NOTIF filter can override any
> * SECCOMP_IOCTL_NOTIF_SEND from earlier filters, essentially allowing all

My takeaway from Chritian's comments is that this comment in the kernel
source is partially wrong, since it is not possible to install multiple
filters with SECCOMP_RET_USER_NOTIF, right?

> * such filtered syscalls to be executed by sending the response
> In other words, from a security perspective, you must assume that the
> target process can bypass any SECCOMP_RET_USER_NOTIF (or
> SECCOMP_RET_TRACE) filters unless it is completely prohibited from
> calling seccomp().

Drawing on text from Chrstian's comment in seccomp.h and Kees's mail,
I added the following in NOTES:

The intent of the user-space notification feature is to allow sys‐
tem calls to be performed on behalf of the target. The target's
system call should either be handled by the supervisor or allowed
to continue normally in the kernel (where standard security poli‐
cies will be applied).

Note well: this mechanism must not be used to make security policy
decisions about the system call, which would be inherently race-
prone for reasons described next.

The SECCOMP_USER_NOTIF_FLAG_CONTINUE flag must be used with cau‐
tion. If set by the supervisor, the target's system call will
continue. However, there is a time-of-check, time-of-use race
here, since an attacker could exploit the interval of time where
the target is blocked waiting on the "continue" response to do
things such as rewriting the system call arguments.

Note furthermore that a user-space notifier can be bypassed if the
existing filters allow the use of seccomp(2) or prctl(2) to
install a filter that returns an action value with a higher prece‐
dence than SECCOMP_RET_USER_NOTIF (see seccomp(2)).

It should thus be absolutely clear that the seccomp user-space
notification mechanism can not be used to implement a security
policy! It should only ever be used in scenarios where a more
privileged process supervises the system calls of a lesser privi‐
leged target to get around kernel-enforced security restrictions
when the supervisor deems this safe. In other words, in order to
continue a system call, the supervisor should be sure that another
security mechanism or the kernel itself will sufficiently block
the system call if its arguments are rewritten to something

Seem okay?

> This should also be noted over in the main
> seccomp(2) manpage, especially the SECCOMP_RET_TRACE part.

I added some words in seccomp(2) to emphasize this.

> [...]
>> This program can used to demonstrate various aspects of the
> nit: "can be used to demonstrate", or alternatively just "demonstrates"

Thanks. Fixed (added "to")

>> behavior of the seccomp user-space notification mechanism. To
>> help aid such demonstrations, the program logs various messages
>> to show the operation of the target process (lines prefixed "T:")
>> and the supervisor (indented lines prefixed "S:").
> [...]
>> Program source
> [...]
>> #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
>> } while (0)
> Don't we have err() for this?

I tend to avoid the use of err() because it's a nonstandard BSDism.
Perhaps by this point this is as much a habit as anything rational.

>> /* Send the file descriptor 'fd' over the connected UNIX domain socket
>> 'sockfd'. Returns 0 on success, or -1 on error. */
>> static int
>> sendfd(int sockfd, int fd)
>> {
>> struct msghdr msgh;
>> struct iovec iov;
>> int data;
>> struct cmsghdr *cmsgp;
>> /* Allocate a char array of suitable size to hold the ancillary data.
>> However, since this buffer is in reality a 'struct cmsghdr', use a
>> union to ensure that it is suitable aligned. */
> nit: suitably

Thanks. Fixed.

>> union {
>> char buf[CMSG_SPACE(sizeof(int))];
>> /* Space large enough to hold an 'int' */
>> struct cmsghdr align;
>> } controlMsg;
>> /* The 'msg_name' field can be used to specify the address of the
>> destination socket when sending a datagram. However, we do not
>> need to use this field because 'sockfd' is a connected socket. */
>> msgh.msg_name = NULL;
>> msgh.msg_namelen = 0;
>> /* On Linux, we must transmit at least one byte of real data in
>> order to send ancillary data. We transmit an arbitrary integer
>> whose value is ignored by recvfd(). */
>> msgh.msg_iov = &iov;
>> msgh.msg_iovlen = 1;
>> iov.iov_base = &data;
>> iov.iov_len = sizeof(int);
>> data = 12345;
>> /* Set 'msghdr' fields that describe ancillary data */
>> msgh.msg_control = controlMsg.buf;
>> msgh.msg_controllen = sizeof(controlMsg.buf);
>> /* Set up ancillary data describing file descriptor to send */
>> cmsgp = CMSG_FIRSTHDR(&msgh);
>> cmsgp->cmsg_level = SOL_SOCKET;
>> cmsgp->cmsg_type = SCM_RIGHTS;
>> cmsgp->cmsg_len = CMSG_LEN(sizeof(int));
>> memcpy(CMSG_DATA(cmsgp), &fd, sizeof(int));
>> /* Send real plus ancillary data */
>> if (sendmsg(sockfd, &msgh, 0) == -1)
>> return -1;
>> return 0;
>> }
> Instead of using unix domain sockets to send the fd to the parent, I
> think you could also use clone3() with flags==CLONE_FILES|SIGCHLD,
> dup2() the seccomp fd to an fd that was reserved in the parent, call
> unshare(CLONE_FILES) in the child after setting up the seccomp fd, and
> wake up the parent with something like pthread_cond_signal()? I'm not
> sure whether that'd look better or worse in the end though, so maybe
> just ignore this comment.

Ahh -- nice. That answers in detail a question I also had for Tycho.
I won't make any changes to the page (since I'm not sure it would
look better), but I will add that detail in a comment in the page
source. Perhaps I'll do something with that in the future.

> [...]
>> /* Access the memory of the target process in order to discover the
>> pathname that was given to mkdir() */
>> static void
>> getTargetPathname(struct seccomp_notif *req, int notifyFd,
>> char *path, size_t len)
>> {
>> char procMemPath[PATH_MAX];
>> snprintf(procMemPath, sizeof(procMemPath), "/proc/%d/mem", req->pid);
>> int procMemFd = open(procMemPath, O_RDONLY);
> Should example code like this maybe use O_CLOEXEC unless the fd in
> question actually has to be inheritable? I know it doesn't actually
> matter here, but if this code was used in a multi-threaded context, it
> might.

Yes, good point. I changed this.

>> if (procMemFd == -1)
>> errExit("Supervisor: open");
>> /* Check that the process whose info we are accessing is still alive.
>> If the SECCOMP_IOCTL_NOTIF_ID_VALID operation (performed
>> in checkNotificationIdIsValid()) succeeds, we know that the
>> /proc/PID/mem file descriptor that we opened corresponds to the
>> process for which we received a notification. If that process
>> subsequently terminates, then read() on that file descriptor
>> will return 0 (EOF). */
>> checkNotificationIdIsValid(notifyFd, req->id);
>> /* Seek to the location containing the pathname argument (i.e., the
>> first argument) of the mkdir(2) call and read that pathname */
>> if (lseek(procMemFd, req->data.args[0], SEEK_SET) == -1)
>> errExit("Supervisor: lseek");
>> ssize_t s = read(procMemFd, path, PATH_MAX);
>> if (s == -1)
>> errExit("read");
> Why not pread() instead of lseek()+read()?

No good reason! I changed it to:

/* Read bytes at the location containing the pathname argument
(i.e., the first argument) of the mkdir(2) call */

ssize_t s = pread(procMemFd, path, PATH_MAX, req->data.args[0]);
if (s == -1)

if (s == 0) {
fprintf(stderr, "\tS: pread() of /proc/PID/mem "
"returned 0 (EOF)\n");


>> if (s == 0) {
>> fprintf(stderr, "\tS: read() of /proc/PID/mem "
>> "returned 0 (EOF)\n");
>> exit(EXIT_FAILURE);
>> }
>> if (close(procMemFd) == -1)
>> errExit("close-/proc/PID/mem");
> We should probably make sure here that the value we read is actually
> NUL-terminated?

So, I was curious about that point also. But, (why) are we not
guaranteed that it will be NUL-terminated?

>> }
>> /* Handle notifications that arrive via the SECCOMP_RET_USER_NOTIF file
>> descriptor, 'notifyFd'. */
>> static void
>> handleNotifications(int notifyFd)
>> {
>> struct seccomp_notif_sizes sizes;
>> char path[PATH_MAX];
>> /* For simplicity, we assume that the pathname given to mkdir()
>> is no more than PATH_MAX bytes; but this might not be true. */
> No, it has to be true, otherwise the kernel would fail the syscall if
> it was executing normally.

Yes. I removed that comment.

>> /* Discover the sizes of the structures that are used to receive
>> notifications and send notification responses, and allocate
>> buffers of those sizes. */
>> if (seccomp(SECCOMP_GET_NOTIF_SIZES, 0, &sizes) == -1)
>> errExit("\tS: seccomp-SECCOMP_GET_NOTIF_SIZES");
>> struct seccomp_notif *req = malloc(sizes.seccomp_notif);
>> if (req == NULL)
>> errExit("\tS: malloc");
>> struct seccomp_notif_resp *resp = malloc(sizes.seccomp_notif_resp);
> This should probably do something like max(sizes.seccomp_notif_resp,
> sizeof(struct seccomp_notif_resp)) in case the program was built
> against new UAPI headers that make struct seccomp_notif_resp big, but
> is running under an old kernel where that struct is still smaller?

I'm confused. Why? I mean, if the running kernel says that it expects
a buffer of a certain size, and we allocate a buffer of that size,
what's the problem?

>> if (resp == NULL)
>> errExit("\tS: malloc");
> [...]
>> } else {
>> /* If mkdir() failed in the supervisor, pass the error
>> back to the target */
>> resp->error = -errno;
>> printf("\tS: failure! (errno = %d; %s)\n", errno,
>> strerror(errno));
>> }
>> } else if (strncmp(path, "./", strlen("./")) == 0) {
> nit: indent messed up

Thanks. Fixed.

And thanks again for the detailed review, Jann.



Michael Kerrisk
Linux man-pages maintainer;
Linux/UNIX System Programming Training:

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