The following changes since commit 7fef2edf7cc753b51f7ccc74993971b0a9c81eca:
sd: don't mess with SD_MINORS for CONFIG_DEBUG_BLOCK_EXT_DEVT (2021-07-12 12:25:37 -0700)
are available in the Git repository at:
git://git.infradead.org/users/hch/configfs.git tags/configfs-5.13-1
for you to fetch changes up to 420405ecde061fde76d67bd3a67577a563ea758e:
configfs: fix the read and write iterators (2021-07-13 20:56:24 +0200)
----------------------------------------------------------------
configfs fix for Linux 5.14
- fix the read and write iterators (Bart Van Assche)
----------------------------------------------------------------
Bart Van Assche (1):
configfs: fix the read and write iterators
fs/configfs/file.c | 29 ++++++++++++++++++++++-------
1 file changed, 22 insertions(+), 7 deletions(-)
On Wed, Jul 14, 2021 at 9:33 AM Christoph Hellwig <[email protected]> wrote:
>
> configfs fix for Linux 5.14
>
> - fix the read and write iterators (Bart Van Assche)
I've pulled this, but I'm somewhat disgusted by it.
The overflow "protection" is just wrong:
+ to_copy = SIMPLE_ATTR_SIZE - 1 - pos;
+ if (to_copy <= 0)
+ return 0;
because if users control "pos", then that "to_copy" could be a huge
positive value even after overflow protection.
I hope/think that we always end up checking 'pos' in the VFS layer so
that this isn't a bug in practice, but people - the above is just
fundamentally bad code.
It's simply not the correct way to check limits. It does it badly, and
it's hard to read (*).
If you want to check limits, then do it (a) the obvious way and (b) right.
Something like
if (pos < 0 || pos >= SIMPLE_ATTR_SIZE - 1)
return 0;
to_copy = SIMPLE_ATTR_SIZE - 1 - pos;
would have been a hell of a lot more obvious, would have been CORRECT,
and a compiler would likely be able to equally good code for it.
Doing a "x <0 || x > C" test is actually nice and cheap, and compilers
should all be smart enough to turn it into a single (unsigned)
comparison.
Possibly it even generates better code, since "to_copy" could then -
and should - no longer be a 64-bit loff_t, since it's pointless. We've
just checked the range of the values, so it can be the natural size
for the machine.
Although from a small test, gcc does seem to be too simple to take
advantage of that, and on 32-bit x86 it does the range check using
64-bit arithmetic even when unnecessary (it should just check "are the
upper 32 bits zero" rather than play around with doing a 64-bit
sub/sbb - I'm surprised, because I thought gcc already knew about
this, but maybe compiler people are starting to forget about 32-bit
stuff too).
But even if the compiler doesn't figure it out, the simple "just check
the limits" is a lot more readable for humans, and avoids the whole
overflow issue. And maybe some compilers will do better at it.
Linus
(*) Ok, it's easy to read if you ignore the overflow possibility. IOW,
it's easy to read WRONG.
On Wed, Jul 14, 2021 at 1:05 PM Linus Torvalds
<[email protected]> wrote:
>
> I hope/think that we always end up checking 'pos' in the VFS layer so
> that this isn't a bug in practice
Yeah, we seem to make sure everything is fine in rw_verify_area().
We do allow negative 'pos' things, but only for files marked with
FMODE_UNSIGNED_OFFSET, which is basically just for variations of
/dev/mem and /proc/<pid>/mem that need the whole 64-bit range.
So it _shouldn't_ be an issue here, but the points about just doing
the legible and safe version stands.
Linus
The pull request you sent on Wed, 14 Jul 2021 18:33:07 +0200:
> git://git.infradead.org/users/hch/configfs.git tags/configfs-5.13-1
has been merged into torvalds/linux.git:
https://git.kernel.org/torvalds/c/1013d4add290c460b816fc4b1db5174f88b71760
Thank you!
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