fallocate on 32 bit boundary on 32 bit systems with setrlimit fails to generate SIGXFSZ signal
Affects | Status | Importance | Assigned to | Milestone | |
---|---|---|---|---|---|
Linux |
Confirmed
|
Medium
|
|||
linux (Ubuntu) |
New
|
High
|
Unassigned |
Bug Description
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.
Changed in linux: | |
importance: | Unknown → Medium |
status: | Unknown → Confirmed |
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
and then change the status of the bug to 'Confirmed'.
If, due to the nature of the issue you have encountered, you are unable to run this command, please add a comment stating that fact and change the bug status to 'Confirmed'.
This change has been made by an automated script, maintained by the Ubuntu Kernel Team.