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SubjectRe: [PATCH] utimensat() non-conformances and fixes -- version 2
From
Miklos,

Thanks for the comments.

Miklos Szeredi wrote:
>> This is a revised version of my earlier
>> (http://article.gmane.org/gmane.linux.man/129/ ) patch to fix
>> non-conformances in the utimensat() implementation.
>
> The patch looks functionally correct. But there are several things I
> don't really like...

[..]

>> - if (!times || (nsec_special(times[0].tv_nsec) &&
>> - nsec_special(times[1].tv_nsec))) {
>> + if (!times || (times[0].tv_nsec == UTIME_NOW &&
>> + times[1].tv_nsec == UTIME_NOW)) {
>> error = -EACCES;
>> +
>
> How about explicitly turning UTIME_NOW/UTIME_NOW into times = NULL at
> the beginning of the function? That would both simplify things and
> also make it absolutely sure that the two cases are handled the same
> way (which makes sense, and is also what the standard wants).

Yes, simple, and sensible.

>> if (IS_IMMUTABLE(inode))
>> goto mnt_drop_write_and_out;
>>
>> @@ -147,7 +145,20 @@
>> goto mnt_drop_write_and_out;
>> }
>> }
>> + } else if (times && ((times[0].tv_nsec == UTIME_NOW &&
>> + times[1].tv_nsec == UTIME_OMIT)
>> + ||
>> + (times[0].tv_nsec == UTIME_OMIT &&
>> + times[1].tv_nsec == UTIME_NOW))) {
>> + error = -EPERM;
>> +
>> + if (IS_IMMUTABLE(inode) || IS_APPEND(inode))
>> + goto mnt_drop_write_and_out;
>> +
>> + if (!is_owner_or_cap(inode))
>> + goto mnt_drop_write_and_out;
>
> I don't like adding _more_ owner checks to this function. It would be
> better if we were removing them: some weird filesystems want to do
> their own permission checking and so the owner checks should really be
> moved into inode_change_ok().
>
> One way to do that could be to add a pseudo attribute flag,
> e.g. ATTR_TIMES_UPDATE, that tells the permission checking code to
> check the owner, even when neither ATTR_[AM]TIME_SET flag is set.
>
> Even the check for the owner in the !times case could be removed, by
> adding ATTR_TIMES_UPDATE only if we don't have write permission on the
> file. That's a cleanup I'd really be happy with.

See below.

> All this may also be done with the ATTR_FORCE flag, but that would
> mean:
>
> - modifying lots of call sites
> - making it impossible to selectively check the permission if
> multiple attributes are being modified (don't know if any callers
> want that though).

Okay, I won't go that route...

Regarding your suggestions above, are you meaning something like the
patch below?

The patch is a little less pretty than I'd like because of the need to
return EACCES or EPERM depending on whether (times == NULL). In
particular, these lines in utimes.c:

+ if (!times && error == -EPERM)
+ error = -EACCES;
seem a little fragile (but maybe I worry too much).

Cheers,

Michael

========================
diff -ru linux-2.6.26-rc2/fs/attr.c linux-2.6.26-rc2-utimensat-fix/fs/attr.c
--- linux-2.6.26-rc2/fs/attr.c 2008-01-24 23:58:37.000000000 +0100
+++ linux-2.6.26-rc2-utimensat-fix/fs/attr.c 2008-05-16 21:56:51.000000000 +0200
@@ -51,7 +51,8 @@
}

/* Check for setting the inode time. */
- if (ia_valid & (ATTR_MTIME_SET | ATTR_ATIME_SET)) {
+ if (ia_valid & (ATTR_MTIME_SET | ATTR_ATIME_SET |
+ ATTR_TIMES_UPDATE)) {
if (!is_owner_or_cap(inode))
goto error;
}
diff -ru linux-2.6.26-rc2/fs/utimes.c linux-2.6.26-rc2-utimensat-fix/fs/utimes.c
--- linux-2.6.26-rc2/fs/utimes.c 2008-05-15 10:33:20.000000000 +0200
+++ linux-2.6.26-rc2-utimensat-fix/fs/utimes.c 2008-05-16 22:14:31.000000000 +0200
@@ -40,14 +40,9 @@

#endif

-static bool nsec_special(long nsec)
-{
- return nsec == UTIME_OMIT || nsec == UTIME_NOW;
-}
-
static bool nsec_valid(long nsec)
{
- if (nsec_special(nsec))
+ if (nsec == UTIME_OMIT || nsec == UTIME_NOW)
return true;

return nsec >= 0 && nsec <= 999999999;
@@ -102,11 +97,15 @@
if (error)
goto dput_and_out;

+ if (times && times[0].tv_nsec == UTIME_NOW &&
+ times[1].tv_nsec == UTIME_NOW)
+ times = NULL;
+
/* Don't worry, the checks are done in inode_change_ok() */
newattrs.ia_valid = ATTR_CTIME | ATTR_MTIME | ATTR_ATIME;
if (times) {
error = -EPERM;
- if (IS_APPEND(inode) || IS_IMMUTABLE(inode))
+ if (IS_APPEND(inode) || IS_IMMUTABLE(inode))
goto mnt_drop_write_and_out;

if (times[0].tv_nsec == UTIME_OMIT)
@@ -131,25 +130,39 @@
* UTIME_NOW, then need to check permissions, because
* inode_change_ok() won't do it.
*/
- if (!times || (nsec_special(times[0].tv_nsec) &&
- nsec_special(times[1].tv_nsec))) {
+ if (!times) {
error = -EACCES;
if (IS_IMMUTABLE(inode))
goto mnt_drop_write_and_out;

- if (!is_owner_or_cap(inode)) {
- if (f) {
- if (!(f->f_mode & FMODE_WRITE))
- goto mnt_drop_write_and_out;
- } else {
- error = vfs_permission(&nd, MAY_WRITE);
- if (error)
- goto mnt_drop_write_and_out;
- }
+ if (f) {
+ if (!(f->f_mode & FMODE_WRITE))
+ newattrs.ia_valid |= ATTR_TIMES_UPDATE;
+ } else {
+ error = vfs_permission(&nd, MAY_WRITE);
+ if (error == -EACCES)
+ newattrs.ia_valid |= ATTR_TIMES_UPDATE;
+ else if (error)
+ goto mnt_drop_write_and_out;
}
+ } else if (times && ((times[0].tv_nsec == UTIME_NOW &&
+ times[1].tv_nsec == UTIME_OMIT)
+ ||
+ (times[0].tv_nsec == UTIME_OMIT &&
+ times[1].tv_nsec == UTIME_NOW))) {
+ error = -EPERM;
+
+ if (IS_IMMUTABLE(inode) || IS_APPEND(inode))
+ goto mnt_drop_write_and_out;
+
+ newattrs.ia_valid |= ATTR_TIMES_UPDATE;
}
+
mutex_lock(&inode->i_mutex);
error = notify_change(dentry, &newattrs);
+ if (!times && error == -EPERM)
+ error = -EACCES;
+
mutex_unlock(&inode->i_mutex);
mnt_drop_write_and_out:
mnt_drop_write(mnt);
diff -ru linux-2.6.26-rc2/include/linux/fs.h linux-2.6.26-rc2-utimensat-fix/include/linux/fs.h
--- linux-2.6.26-rc2/include/linux/fs.h 2008-05-15 10:33:25.000000000 +0200
+++ linux-2.6.26-rc2-utimensat-fix/include/linux/fs.h 2008-05-16 21:39:24.000000000 +0200
@@ -317,22 +317,23 @@
* Attribute flags. These should be or-ed together to figure out what
* has been changed!
*/
-#define ATTR_MODE 1
-#define ATTR_UID 2
-#define ATTR_GID 4
-#define ATTR_SIZE 8
-#define ATTR_ATIME 16
-#define ATTR_MTIME 32
-#define ATTR_CTIME 64
-#define ATTR_ATIME_SET 128
-#define ATTR_MTIME_SET 256
-#define ATTR_FORCE 512 /* Not a change, but a change it */
-#define ATTR_ATTR_FLAG 1024
-#define ATTR_KILL_SUID 2048
-#define ATTR_KILL_SGID 4096
-#define ATTR_FILE 8192
-#define ATTR_KILL_PRIV 16384
-#define ATTR_OPEN 32768 /* Truncating from open(O_TRUNC) */
+#define ATTR_MODE 1
+#define ATTR_UID 2
+#define ATTR_GID 4
+#define ATTR_SIZE 8
+#define ATTR_ATIME 16
+#define ATTR_MTIME 32
+#define ATTR_CTIME 64
+#define ATTR_ATIME_SET 128
+#define ATTR_MTIME_SET 256
+#define ATTR_FORCE 512 /* Not a change, but a change it */
+#define ATTR_ATTR_FLAG 1024
+#define ATTR_KILL_SUID 2048
+#define ATTR_KILL_SGID 4096
+#define ATTR_FILE 8192
+#define ATTR_KILL_PRIV 16384
+#define ATTR_OPEN 32768 /* Truncating from open(O_TRUNC) */
+#define ATTR_TIMES_UPDATE 65536

/*
* This is the Inode Attributes structure, used for notify_change(). It

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