Hi,
I was attempting to run the firmware upload selftests in the linux
kernel source on a Fedora 43 system, and I discovered that the tests
were failing when comparing expected firmware data against the data
reported by sysfs.
Further, I discovered that the tests are only failed when using
"cmp -s" or when redirecting the output to /dev/null.
Using GDB, I found that it is failing in the code block that I have
pasted below. The comments indicate that special handling is required
for /proc files because they don't return that actual file size.
Similar handling may be required for sysfs files, which typically
returns 4096 for the page size:
$ ls -l /sys/devices/virtual/misc/test_firmware/upload_read
-r--r--r-- 1 root root 4096 Apr 8 13:45
/sys/devices/virtual/misc/test_firmware/upload_read
The version of cmp is 3.12 (with a couple of fedora patches added)
Thanks,
- Russ Weight
342 /* If no output is needed,
343 and both input descriptors are associated with plain files,
344 and the file sizes are nonzero so they are not Linux /proc files,
345 conclude that the files differ if they have different sizes
346 and if more bytes will be compared than are in the smaller file. */
347
348 if (type_no_stdout <= comparison_type
349 && 0 <= stat_buf[0].st_size && S_ISREG (stat_buf[0].st_mode)
350 && 0 <= stat_buf[1].st_size && S_ISREG (stat_buf[1].st_mode))
351 {
352 off_t pos0 = file_position (0);
353 if (0 <= pos0)
354 {
355 off_t pos1 = file_position (1);
356 if (0 <= pos1)
357 {
358 off_t s0 = stat_buf[0].st_size - pos0;
359 off_t s1 = stat_buf[1].st_size - pos1;
360 if (s0 < 0)
361 s0 = 0;
362 if (s1 < 0)
363 s1 = 0;
364 if (s0 != s1 && MIN (s0, s1) < bytes)
365 exit (EXIT_FAILURE);
366 }
367 }
368 }
