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SubjectRe: [PATCHv3 man-pages 3/3] open.2: describe O_BENEATH flag
On Mon, Mar 9, 2015 at 2:32 PM, Michael Kerrisk (man-pages)
<mtk.manpages@gmail.com> wrote:
> On 03/09/2015 03:00 PM, David Drysdale wrote:
>> Signed-off-by: David Drysdale <drysdale@google.com>
>
> Hi David,
>
> The text looks good insofar as it goes. But, it would be helpful
> to have sentence or to that explains why this flag exists.
> Could you add that, please?
>
> Thanks,
>
> Michael

How about something like:

This feature allows applications to be sure that the opened file
is within the specified directory, regardless of the original
source of the pathname argument. Some security-conscious pro‐
grams may further ensure this by imposing a system call filter
(with seccomp(2)) that requires this flag for all open() opera‐
tions, so that the program cannot open files outside of speci‐
fied directories even if subverted.

(Also, I realize that I somehow failed to notice that the flags
are listed in alphabetical order, so I'll move the text up, as
in the updated diff below).

Thanks,
David

---
man2/open.2 | 44 ++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 44 insertions(+)

diff --git a/man2/open.2 b/man2/open.2
index 956531b24b26..ece1fa90775a 100644
--- a/man2/open.2
+++ b/man2/open.2
@@ -201,6 +201,43 @@ See
for further details.
See also BUGS, below.
.TP
+.B O_BENEATH " (since Linux 4.??)"
+Ensure that the
+.I pathname
+is beneath the current working directory (for
+.BR open (2))
+or the
+.I dirfd
+(for
+.BR openat (2)).
+If the
+.I pathname
+is absolute or contains a path component of "..", the
+.BR open ()
+fails with the error
+.BR EPERM.
+This occurs even if ".." path component would not actually
+escape the original directory; for example, a
+.I pathname
+of "subdir/../filename" would be rejected.
+Path components that are symbolic links to absolute paths, or that are
+relative paths containing a ".." component, will also cause the
+.BR open ()
+operation to fail with the error
+.BR EPERM.
+
+This feature allows applications to be sure that the opened file is
+within the specified directory, regardless of the original source of the
+.I pathname
+argument.
+Some security-conscious programs may further ensure
+this by imposing a system call filter (with
+.BR seccomp (2))
+that requires this flag for all
+.BR open ()
+operations, so that the program cannot open files outside of
+specified directories even if subverted.
+.TP
.BR O_CLOEXEC " (since Linux 2.6.23)"
.\" NOTE! several other man pages refer to this text
Enable the close-on-exec flag for the new file descriptor.
@@ -984,6 +1021,13 @@ did not match the owner of the file and the
caller was not privileged
The operation was prevented by a file seal; see
.BR fcntl (2).
.TP
+.B EPERM
+The
+.B O_BENEATH
+flag was specified and the
+.I pathname
+was not beneath the relevant directory.
+.TP
.B EROFS
.I pathname
refers to a file on a read-only filesystem and write access was
--
2.2.0.rc0.207.ga3a616c

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