Date: Wed, 02 Jan 2008 22:38:40 -0800 From: Jason Evans <jasone@freebsd.org> To: freebsd-current@freebsd.org Cc: Poul-Henning Kamp <phk@FreeBSD.org> Subject: sbrk(2) broken Message-ID: <477C82F0.5060809@freebsd.org>
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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. Thanks, Jason
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