Running command 'cat /proc/<pid>/stat | cut -d" " -f29' to get stack pointer , is always showing stack pointer as zero

Question

Iam trying to get the stack pointer of some thread using the /proc//stat, whenever i run the command,cat /proc/<pid>/stat | cut -d" " -f29 i end up getting zero, but when i run sudo cat /proc/<pid>/stack i do end up getting a value

[<0>] worker_thread+0xb7/0x390
[<0>] kthread+0x134/0x150
[<0>] ret_from_fork+0x1f/0x40

not sure this is happening, any ideas why the stack pointer keeps being zero, thanks.

Answer 1

The fields are intentionally zeroed out. Quote fs/proc/array.c:

/*
 * esp and eip are intentionally zeroed out.  There is no
 * non-racy way to read them without freezing the task.
 * Programs that need reliable values can use ptrace(2).
 *
 * The only exception is if the task is core dumping because
 * a program is not able to use ptrace(2) in that case. It is
 * safe because the task has stopped executing permanently.
 */

This zeroing out can be seen at line 489 in the same file from the current Linux 6.10-rc2:

vsize = eip = esp = 0;