[linux] fix: memory-mapped file, stat: No such file or directory

[jones2024gh]

Describe the bug
Since lsof-4.96.0, run lsof with old linux kernel (e.g. linux-2.6),
lsof would print error message "stat: No such file or directory"
when visiting memory-mapped files; but /bin/stat could visit it without issue.
lsof-4.95.0 and earlier does not have this issue.

• 4.99.4 fail
• 4.99.3 fail
• 4.96.0 fail
• 4.95.0 pass
• 4.92.0 pass

In old linux kernel (e.g. linux-2.6) which does not have this feature: Commit
6b4e306aa3dc ("ns: proc files for namespace naming policy."), means this path
"/proc/self/ns" is not existed. Since lsof-4.96.0 with Commit dbad150 ("
[linux] obtain correct information of memory-mapped file."), compare_mntns()
would misunderstand it is in a different mount namespace if "/proc/self/ns" is
not existed, returns -1, go through map_files lookup, and finally lead to lsof
gets failed with error message "stat: No such file or directory".

If "/proc/self/ns" or its underlying path is not existed, compare_mntns()
returns 0 instead of -1, in order to go through stat_directly as old days.

To Reproduce
Steps to reproduce the behavior:

1. Run 'lsof -p 1'
2. See error

Expected behavior
Should not print 'stat: No such file or directory'.

Program output

# lsof -p 1
COMMAND PID USER  FD   TYPE DEVICE SIZE/OFF NODE NAME
init      1 user cwd    DIR   31,6      339  352 /
init      1 user rtd    DIR   31,6      339  352 /
init      1 user txt    REG   31,6   694544 5323 /bin/busybox
init      1 user mem    REG   31,6          4499 /lib/ld-uClibc-0.9.33.2.so (stat: No such file or directory)
init      1 user mem    REG   31,6          4400 /lib/libcrypt-0.9.33.2.so (stat: No such file or directory)
init      1 user mem    REG   31,6          4446 /lib/public/liblogintoken.so (stat: No such file or directory)
init      1 user mem    REG   31,6          4374 /lib/libm-0.9.33.2.so (stat: No such file or directory)
init      1 user mem    REG   31,6          4343 /lib/libuClibc-0.9.33.2.so (stat: No such file or directory)
init      1 user mem    REG   31,6          4434 /lib/libgcc_s.so.1 (stat: No such file or directory)
init      1 user   0u   CHR    5,1      0t0   17 /dev/console
init      1 user   1u   CHR    5,1      0t0   17 /dev/console
init      1 user   2u   CHR    5,1      0t0   17 /dev/console
# /bin/stat /lib/ld-uClibc-0.9.33.2.so
  File: "/lib/ld-uClibc-0.9.33.2.so"
  Size: 31568           Blocks: 62         IO Block: 1024   regular file
Device: 1f06h/7942d     Inode: 4499        Links: 1
Access: (0755/-rwxr-xr-x)  Uid: (    0/ test)   Gid: (    0/    root)
Access: 2024-12-16 21:51:20.000000000
Modify: 2024-12-16 21:51:20.000000000
Change: 2024-12-16 21:51:20.000000000

Environment (please complete the following information):

• Kernel: linux-2.6.36
• OS: embedded linux with customized build
• lsof Version: 4.99.4

[jiegec]

Nice observation! Regarding the code change, maybe we could add some comments on the logic behind comparison: when it is regarded as different, and how it works in different kernel versions.

[jones2024gh]

Hello @jiegec

Thank you for kind feedback! Add comments and hope im doing this in the correct way :)

[jones2024gh]

Thank you @jiegec and your time :)