Date: Thu, 3 Jan 2008 22:39:36 +0200 From: Kostik Belousov <kostikbel@gmail.com> To: Jason Evans <jasone@freebsd.org> Cc: freebsd-current@freebsd.org Subject: Re: sbrk(2) broken Message-ID: <20080103203936.GX57756@deviant.kiev.zoral.com.ua> In-Reply-To: <477C82F0.5060809@freebsd.org> References: <477C82F0.5060809@freebsd.org>
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On Wed, Jan 02, 2008 at 10:38:40PM -0800, Jason Evans wrote:
> Poul-Henning noticed today that xchat fails to start if malloc uses sbrk
> internally. This failure happens during the first call to malloc, with
> the following message:
>
> Fatal error 'Can't allocate initial thread' at line 335 in file
> /usr/src/lib/libthr/thread/thr_init.c (errno = 12)
>
> This can be worked around with MALLOC_OPTIONS=dM .
>
> The problem does not appear to be specific to jemalloc; I reverted
> src/lib/libc/stdlib/malloc.c to revision 1.92 (last phkmalloc revision),
> which also uses sbrk, and the failure mode is the same.
>
> The failure occurs on both i386 and amd64. It appears that sbrk(0)
> returns an address that is in the address range normally used by mmap.
> So, the first call to sbrk with a non-zero increment is fantastically
> wrong. On i386 (ktrace output):
>
> 1013 xchat CALL break(0x28200000)
> 1013 xchat RET break -1 errno 12 Cannot allocate memory
>
> On amd64 (truss ouput):
>
> break(0x800900000) ERR#12 'Cannot allocate memory'
>
> sbrk is not a true system call, so it seems like the problem should have
> something to do with the _end data symbol. I looked at it in gdb though
> and never saw an unreasonable value, despite bogus sbrk(0) results. I
> do not know offhand how to get the addresses of .minbrk and .curbrk
> (register inspection within gdb while stepping through sbrk?), which are
> what sbrk actually uses (see src/lib/libc/amd64/sys/sbrk.S). Perhaps
> the loader isn't initializing them correctly...
>
> I am quite pressed for time at the moment, and cannot look into this in
> any more detail for at least a couple of weeks. If anyone knows what
> the problem is, please let me know.
I cannot say definitely what happen, but please note that the _end
symbol is defined by linker script, and it shall be present in all
executable and shared objects. The value you reported would be naturally
the _end value for some shared object.
I tried both the RELENG_7 and HEAD, and sbrk(0) correctly returns a
seemingly valid value like 0x8049644.
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
int
main(int argc, char *argv)
{
void *p;
p = sbrk(0);
printf("%p\n", p);
return (0);
}
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