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Sat, 22 Dec 2018 13:03:35 -0800

On 2018-12-22 19:42, Tim R¨¹hsen wrote:
> On 22.12.18 16:56, Aurelien Jarno wrote:
> > The problem is that qemu-arm does not provide a heap to the program, so
> > glibc fails to alloc memory through brk. This causes malloc to switch to
> > mmap based memory allocation, and this also sets errno to ENOMEM.
> > 
> > printf also calls malloc, so the malloc implementation switches to
> > mmap based memory allocation at this moment. This is remembered through
> > the life of the program. When strerror then calls malloc(1024), the
> > allocation is done directly through mmap and errno is not set to ENOMEM.

Setting errno to ENOMEM happens during the call to MORECORE/sbrk in 
malloc/malloc.c in the following lines of the sysmalloc function:

|      if (size > 0)
|        {
|          brk = (char *) (MORECORE (size));
|          LIBC_PROBE (memory_sbrk_more, 2, brk, size);
|        }

Note however that clobbering errno is allowed even in case of success,
unless the contrary is specifically documented.

> > That's why you do not see the issue.
> > 
> > To reproduce the issue, you therefore need the following conditions:
> > - The kernel or QEMU does not provide a heap to the program
> > - malloc is not called (implicitly or explicitly) before the call to
> >   strerror
> > - strerror is called with an invalid error number.
> > 
> > If all of this 3 conditions are not met, the bug does not appear.
> 
> That is a good explanation and makes sense to me, thank you, Aurelien.
> 
> At least we can work around that issue now.
> 
> BTW, how do you debug cross-compiled executables ? There is no cross-gdb
> packaged in debian (or is there ?). Building that from scratch seems too
> time-intensive...

I have been rebuilding glibc, directly modifying the code being
debugged. ccache is really useful in that case.

Regards,
Aurelien

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
aurel...@aurel32.net                 free online bettinghttp://www.aurel32.net

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