Re: Ambigiuous thread stack annotation in /proc/pid/[s]maps

[Jan Glauber]

On Thu, Jun 27, 2013 at 10:00:51PM +0530, Siddhesh Poyarekar wrote:
> On 27 June 2013 21:32, Jan Glauber <jan.glauber@xxxxxxxxx> wrote:
> > But isn't that confusing to the user? At least it is to me. Imagine someone
> > who uses the maps or smaps output to determine the size of code, data and
> > stack of a process. Maybe it would be better to not print the stack:tid data
> > at all if the kernel cannot distinguish the vma's?
> > Is that behaviour documented anywhere?
> 
> I'm afraid the documentation update I made to proc.txt did not mention
> this.  In fact I went through the discussion thread and I don't think
> I or anyone mentioned this in the thread either.  I think I assumed
> back then that this was accepted since there was a point made that
> vmas used by makecontext/swapcontext should also get marked correctly
> with [stack].
> 
> However, I agree that you have a point about it being misleading.
> Avoiding a vma merge is a possible solution, but we don't have flags
> available any more to do that.  I'll try to think of another way.  In
> the mean time I could add a note to the proc.txt documentation and
> even adjust the language.  Would that be good enough or do you think
> the patch should be reverted until I or someone else comes up with a
> better solution?

I think we should try to solve the problem first. If it turns out that
it is not possible to prevent these vma merges than we should document it.
Removing the annotation would IMHO be bad since it is really useful
information to the user.

Can anyone from the mm folks comment on the vm_flags situation (CC-ing linux-mm) ?
Would it be possible to re-use one of the defined flags? Or should we come up with a
crude hack?
 
> > Never seen that makecontext stuff before. Do you have an example output how
> > the maps would look like if that is used?
> 
> This is a sample program I cribbed from the makecontext man page and
> modified slightly.  The stack is in the data area in this example.
> There could be a case of a stack being with the main program stack or
> even in the heap.
> 
> #include <ucontext.h>
> #include <stdio.h>
> #include <stdlib.h>
> 
> static ucontext_t uctx_main, uctx_func1, uctx_func2;
> 
> #define handle_error(msg) \
>            do { perror(msg); exit(EXIT_FAILURE); } while (0)
> 
> static char func1_stack[16384];
> static char func2_stack[16384];
> 
> static void func1(void)
> {
>         printf("func1: started\n");
>         printf("func1: swapcontext(&uctx_func1, &uctx_func2)\n");
>         if (swapcontext(&uctx_func1, &uctx_func2) == -1)
>                 handle_error("swapcontext");
> 
>         sleep (1);
>         printf("func1: returning\n");
> }
> 
> static void func2(void)
> {
>         printf("func2: started\n");
>         printf("func2: swapcontext(&uctx_func2, &uctx_func1)\n");
>         if (swapcontext(&uctx_func2, &uctx_func1) == -1)
>                 handle_error("swapcontext");
>         sleep (1);
>         printf("func2: returning\n");
> }
> 
> int main(int argc, char *argv[])
> {
>         if (getcontext(&uctx_func1) == -1)
>                 handle_error("getcontext");
>         uctx_func1.uc_stack.ss_sp = func1_stack;
>         uctx_func1.uc_stack.ss_size = sizeof(func1_stack);
>         uctx_func1.uc_link = &uctx_main;
>         makecontext(&uctx_func1, func1, 0);
> 
>         if (getcontext(&uctx_func2) == -1)
>                 handle_error("getcontext");
>         uctx_func2.uc_stack.ss_sp = func2_stack;
>         uctx_func2.uc_stack.ss_size = sizeof(func2_stack);
>         /* Successor context is f1(), unless argc > 1 */
>         uctx_func2.uc_link = (argc > 1) ? NULL : &uctx_func1;
>         makecontext(&uctx_func2, func2, 0);
> 
>         printf("main: swapcontext(&uctx_main, &uctx_func2)\n");
>         if (swapcontext(&uctx_main, &uctx_func2) == -1)
>                 handle_error("swapcontext");
> 
>         printf("main: exiting\n");
>         exit(EXIT_SUCCESS);
> }
> 
> 
> $./a.out
> $ cat /proc/$(pgrep a.out)/maps
> 
> 00400000-00401000 r-xp 00000000 fd:00 1704114
>   /tmp/a.out
> 00601000-00602000 rw-p 00001000 fd:00 1704114
>   /tmp/a.out
> 00602000-0060a000 rw-p 00000000 00:00 0
>   [stack:7352]
> 7fac662d2000-7fac6647e000 r-xp 00000000 fd:00 524922
>   /usr/lib64/libc-2.15.so
> 7fac6647e000-7fac6667e000 ---p 001ac000 fd:00 524922
>   /usr/lib64/libc-2.15.so
> 7fac6667e000-7fac66682000 r--p 001ac000 fd:00 524922
>   /usr/lib64/libc-2.15.so
> 7fac66682000-7fac66684000 rw-p 001b0000 fd:00 524922
>   /usr/lib64/libc-2.15.so
> 7fac66684000-7fac66689000 rw-p 00000000 00:00 0
> 7fac66689000-7fac666a9000 r-xp 00000000 fd:00 524913
>   /usr/lib64/ld-2.15.so
> 7fac6688f000-7fac66892000 rw-p 00000000 00:00 0
> 7fac668a6000-7fac668a8000 rw-p 00000000 00:00 0
> 7fac668a8000-7fac668a9000 r--p 0001f000 fd:00 524913
>   /usr/lib64/ld-2.15.so
> 7fac668a9000-7fac668aa000 rw-p 00020000 fd:00 524913
>   /usr/lib64/ld-2.15.so
> 7fac668aa000-7fac668ab000 rw-p 00000000 00:00 0
> 7fff5b8a8000-7fff5b8ca000 rw-p 00000000 00:00 0
> 7fff5b9fe000-7fff5ba00000 r-xp 00000000 00:00 0                          [vdso]
> ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0
>   [vsyscall]
> 
> 
> --
> http://siddhesh.in
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