On Wed, 10 Apr 2013 15:11:03 -0400
Jeff King <peff@xxxxxxxx> wrote:
> On Wed, Apr 10, 2013 at 02:51:14PM -0400, Jeff King wrote:
>
> > As for why dmesg reports git-remote-http, I'm not sure. If you
> > "strace -f" the command, you can see that git is running
> > git-remote-https. Why the kernel chooses to report
> > "git-remote-http", I don't know; you'd have to look into how the
> > kernel makes that decision. But I doubt it is related to the reason
> > for the segfault in the first place.
>
> Ah, I see. The hard links are a red herring. The kernel's message uses
My guess was sort of close.
> task->comm, which is presumably set by truncating the basename of the
> program to 15 characters (16 bytes with a trailing NUL).
>
> 3.6 /proc/<pid>/comm & /proc/<pid>/task/<tid>/comm
> --------------------------------------------------------
> These files provide a method to access a tasks comm value. It also
> allows for a task to set its own or one of its thread siblings comm
> value. The comm value is limited in size compared to the cmdline
> value, so writing anything longer then the kernel's TASK_COMM_LEN
> (currently 16 chars) will result in a truncated comm value.
>
> Try:
>
> $ echo 'int main() { sleep(5); *(int *)0=0; }' >foo.c
> $ gcc -o 12345678901234567890 foo.c
> $ ./123* &
> $ cat /proc/$!/comm
> 123456789012345
> $ sleep 5; dmesg | tail -n 1
> [2602639.353584] 123456789012345[23062]: segfault at 0 ip
> 0000000000400524 sp 00007fff46bb0700 error 6 in 12345678901234567890
> [400000+1000]
>
> In both cases we only get the first 15 bytes of the program name. And
> indeed, "git-remote-http" is exactly 15 bytes. So it is dumb luck that
> the limit is such that truncating the name makes it look like another
> program.
I cannot weigh in much on this other then to say it's interesting. I would
think that changing git would be the right thing to do so that proper
reports are made when users encounter errors that the kernel traps.
Maybe making the important part first? https-remote-git?
But I'm certain there's more to it than that!
But then again maybe the kernel should grab the entire command
from /proc/<pid>/cmdline because it's really not providing distinct data.
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