Re: Potentially unbounded allocations in seq_read?

Tvrtko Ursulin
2013-12-11T18:00:04Z

On Wed, 2013-12-11 at 17:49 +0000, Al Viro wrote:
> On Wed, Dec 11, 2013 at 05:04:41PM +0000, Tvrtko Ursulin wrote:
> > Hi all,
> >
> > It seems that the buffer allocation in seq_read can double in size
> > indefinitely, at least I've seen that in practice with /proc/<pid>/smaps
> > (attempting to double m->size to 4M on a read of 1000 bytes). This
> > produces an ugly WARN_ON_ONCE, which should perhaps be avoided? (given
> > that it can be triggered by userspace at will)
>
> An entry in /proc/<pid>/smaps that did not fit into 2Mb? Seriously?
> How in hell has that happened? If you can trigger that at will, please
> post the reproducer.

Yeah, no, wrong assumption. It was not about the size but the number of
reads. For example:

open("/proc/3131/smaps", O_RDONLY|O_LARGEFILE) = 3
fstat64(1, {st_mode=S_IFCHR|0666, st_rdev=makedev(1, 3), ...}) = 0
brk(0xf9019000) = 0xf9019000
read(3, "be483000-be4ab000 rw-s 00000000 "..., 4096) = 4054
write(1, "be483000-be4ab000 rw-s 00000000 "..., 4054) = 4054
read(3, "be5c3000-be5c5000 rw-s 10da3f000"..., 4096) = 3656
write(1, "be5c3000-be5c5000 rw-s 10da3f000"..., 3656) = 3656
read(3, "be6b1000-be6b2000 rw-s 10da48000"..., 4096) = 3661
write(1, "be6b1000-be6b2000 rw-s 10da48000"..., 3661) = 3661
read(3, "be6b9000-be6ba000 rw-s 10da38000"..., 4096) = 3599
write(1, "be6b9000-be6ba000 rw-s 10da38000"..., 3599) = 3599
read(3, "be6d6000-be7b6000 rw-s 10d889000"..., 4096) = 3599
write(1, "be6d6000-be7b6000 rw-s 10d889000"..., 3599) = 3599
read(3, "be884000-be885000 rw-s 10d85c000"..., 4096) = 3661
write(1, "be884000-be885000 rw-s 10d85c000"..., 3661) = 3661
read(3, "be88d000-be8de000 rw-p 00000000 "..., 4096) = 4007
write(1, "be88d000-be8de000 rw-p 00000000 "..., 4007) = 4007
read(3, "bea29000-bea4d000 r-xp 00000000 "..., 4096) = 4057
write(1, "bea29000-bea4d000 r-xp 00000000 "..., 4057) = 4057
read(3, "beab3000-bead0000 r--p 00000000 "..., 4096) = 2092
write(1, "beab3000-bead0000 r--p 00000000 "..., 2092) = 2092
read(3, 0xf9017030, 4096) = -1 ENOMEM (Out of memory)

Regards,
Tvrtko