[minicoredumper] gdb corrupt stack

[John Ogness]

Hi Marco,

On 2017-07-05, Marco.Felsch at preh.de <Marco.Felsch at preh.de> wrote:
> On 03.07.2017 17:06, Marco.Felsch at preh.de wrote:
>> I had cross compiled the minicoredumper for the aarch64.
>> Generating the core file works fine but analyze the the core file
>> with gdb on a remote host isn't possible.
>
> After some debugging I found the problem. Since linux-v4.9 it is not
> possible to get the stack from the /proc/stat file system since commit
> 0a1eb2d47 [0].
>
> So the below code should be changed to use ptrace:
>
> 	/* find 29th item: man proc(5) */
> 	p = buf;
> 	for (i = 0; i < 28; i++) {
> 		p = strchr(p, ' ');
> 		if (!p)
> 			goto out_err;
> 		p++;
> 	}
>
> 	/* read stack pointer */
> 	if (sscanf(p, "%lu ", addr) != 1)
> 		goto out_err;

Thank you for pointing this out. The reason why I implemented the use of
/proc/stat instead of ptrace(2) is because it offered a simple
cross-platform interface to the stack pointer. The kernel has the
convenient KSTK_ESP macro that isn't available to userspace.

I expect we'll have to include our own KSTK_ESP macro to read the
correct register for each platform. :-/

John Ogness

> [0] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/commit/fs/proc/array.c?h=linux-4.9.y&id=0a1eb2d474edfe75466be6b4677ad84e5e8ca3f5