Re: [PATCH v3] malloc: Remove bin scanning from memalign (bug 30723)

[Florian Weimer <fweimer@redhat.com>]

* Stefan Liebler:

>> Does your kernel have a fix for the early sbrk failure?  I suspect what
>> happens is that we fall over to mmap from sbrk, making the heap
>> non-contiguous.
>> 
>> The sbrk failure should be visible under strace.
>> 
> Yes, strace shows mmap with randomize_va_space=0:
> brk(NULL)                               = 0x3fff7fb2000
> brk(0x3fff7fd3000)                      = 0x3fff7fd3000
> brk(0x3fff7ff7000)                      = 0x3fff7fd3000
> mmap(NULL, 1048576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1,
> 0) = 0x3fff7b00000

Ahh, looks like we can get 4 KiB or maybe 8 KiB of heap from the kernel.
That's why we didn't crash before in early startup.  It looks like a
another ASLR layout issue, different from the problem that broke static
PIE.

Not sure what we can do about this.  Obviously, we'd prefer if the test
suite ran with a more production-like malloc configuration.

> write(1, "error: tst-memalign-2.c:158: not"..., 54error:
> tst-memalign-2.c:158: not true: count > LN / 2
> ) = 54
>
> Note that there is this previous mmap-event before set_tid_address-syscall:
> mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0)
> = 0x3fff7ff7000

This is the ld.so minimal malloc, it doesn't use sbrk.

Thanks,
Florian