fallocate on 32 bit boundary on 32 bit systems with setrlimit fails to generate SIGXFSZ signal

[Colin Ian King]

This is a corner case on 32 bit systems when using large file offsets, fallocate and setrlimit.

Setting the RLIMIT_FSIZE with setrlimit to 0xffffffff and then fallocating 1 or more bytes at the offset of 0xffffffff should make the fallocate fail with EFBIG and generate a SIGXFSZ signal. On 64 bit platforms this works, on 32 bit platforms such as i386 Ubuntu bionic with 4.15 kernels it fails to generate EFBIG errors and SIGXFSZ.

Attached is a test program to illustrate the problem. It sets the file size limit and allocates 1024 bytes at the boundary file size limit for 3 offsets:

On 64 bit systems we get the expected results:
got signal SIGXFSZ
offset: 65536 (0x10000), fallocate returned: -1
got signal SIGXFSZ
offset: 4294966271 (0xfffffbff), fallocate returned: -1
got signal SIGXFSZ
offset: 4294967295 (0xffffffff), fallocate returned: -1

On 32 bit systems the code fails on the 0xffffffff offset:

got signal SIGXFSZ
offset: 65536 (0x10000), fallocate returned: -1
got signal SIGXFSZ
offset: 4294966271 (0xfffffbff), fallocate returned: -1
offset: 4294967295 (0xffffffff), fallocate returned: 0

Attached is the reproducer.

I found this while developing a file limit boundary test case in stress-ng and discovered it breaks on all 32 bit kernels (armhf, i386, etc), even with recent 5.15 kernels.

This could be seen as a security issue; the sysadmin can set the file size limit and yet a 32 bit system can use a corner case like this to fallocate a much larger file by using the 0xffffffff offset and a huge fallocate size.

[Ubuntu Kernel Bot (ubuntu-kernel-bot)]

This bug is missing log files that will aid in diagnosing the problem. While running an Ubuntu kernel (not a mainline or third-party kernel) please enter the following command in a terminal window:

apport-collect 1994079

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This change has been made by an automated script, maintained by the Ubuntu Kernel Team.

[Colin Ian King (colin-king)]

occurs on 6.0 i386 kernels too

[colin.i.king (colin.i.king-linux-kernel-bugs)]

Created attachment 303085
C source to reproduce the fallocate/SIGXFSZ issue

This is a corner case on 32 bit systems when using large file offsets, fallocate and setrlimit. Issue found on ext4, probably also on other file systems(?).

Setting the RLIMIT_FSIZE with setrlimit to 0xffffffff and then fallocating 1 or more bytes at the offset of 0xffffffff should make the fallocate fail with EFBIG and generate a SIGXFSZ signal. On 64 bit platforms this works, on 32 bit platforms such as i386 4.15 kernels through to linux 6.0 it fails to generate EFBIG errors and SIGXFSZ.

Attached is a test program to illustrate the problem. It sets the file size limit and allocates 1024 bytes at the boundary file size limit for 3 offsets:

On 64 bit systems we get the expected results:
got signal SIGXFSZ
offset: 65536 (0x10000), fallocate returned: -1
got signal SIGXFSZ
offset: 4294966271 (0xfffffbff), fallocate returned: -1
got signal SIGXFSZ
offset: 4294967295 (0xffffffff), fallocate returned: -1

On 32 bit systems the code fails on the 0xffffffff offset:

got signal SIGXFSZ
offset: 65536 (0x10000), fallocate returned: -1
got signal SIGXFSZ
offset: 4294966271 (0xfffffbff), fallocate returned: -1
offset: 4294967295 (0xffffffff), fallocate returned: 0

Attached is the reproducer.

I found this while developing a file limit boundary test case in stress-ng and discovered it breaks on all 32 bit kernels (armhf, i386, etc), even with recent 5.15 and 6.0 kernels.

attached is a simple reproducer

[colin.i.king (colin.i.king-linux-kernel-bugs)]

Tested and fails also on m68k Linux 4.16

[colin.i.king (colin.i.king-linux-kernel-bugs)]

Tested and fails on m68k with ext4 and btrfs; occurs across arches and across file systems.

[brauner (brauner-linux-kernel-bugs)]

Hey Colin,

This looks to be intentional? Afaict, the -EFBIG might ceome directly from vfs_fallocate():

 /* Check for wrap through zero too */
 if (((offset + len) > inode->i_sb->s_maxbytes) || ((offset + len) < 0))
  return -EFBIG;

and you should see the same behavior for 64bit if you pass in -1 as offset:

brauner@wittgenstein|~/Downloads
> sudo ./fallocate
got signal SIGXFSZ
offset: 65536 (0x10000), fallocate returned: -1
got signal SIGXFSZ
offset: 4294966271 (0xfffffbff), fallocate returned: -1
offset: 18446744073709551615 (0xffffffffffffffff), fallocate returned: -1

[colin.i.king (colin.i.king-linux-kernel-bugs)]

The behavior for 32 bit is not what I expected,

offset: 4294967295 (0xffffffff), fallocate returned: 0

allocating at the boundary is OK, where as at 0xfffffbff it fails. I'd expect at the boundary for it to fail too.