Date: Wed, 14 Sep 2016 16:04:50 +0200 From: u-uy74@...ey.se To: musl@...ts.openwall.com Subject: Re: incompatibility between libtheora/mmx and musl ? On Wed, Sep 14, 2016 at 01:24:00PM +0200, Szabolcs Nagy wrote: > > #define _ogg_malloc(x) malloc((x)+256) > > #define _ogg_calloc(x,y) calloc((x)+256,(y)) > > #define _ogg_realloc(y,x) realloc((y),(x)+256) > > #define _ogg_free free > > > > instead of the default > > > > #define _ogg_malloc malloc > > #define _ogg_calloc calloc > > #define _ogg_realloc realloc > > #define _ogg_free free > > > > did not make any difference. The crash on a test file occurs in the same > > way and the resulting partial output file is as long as otherwise. > > > > This may mean that this is not a simple overflowing but rather > > overwriting or reading distant "random" places (?) (register corruption?) > can be underflow (or the way they align the pointer returned by malloc) Pointer alignment yes they do in some cases but in a different layer, inside the malloc()-ed buffers, it is plain C and looks harmless to me. > you can increase/decrease alignment of musl's alloc by > changing SIZE_ALIGN in src/malloc/malloc.c Doubling the alignment did not apparently change the crashing. Reducing the alignment in half did not apparently change the crashing. (A single test file with a single quality setting tested crashed the same way, at the same place in the output stream) > (or you can try some hack in _ogg_malloc/free if you are > sure that's what they are using) Yes it is present/used for this very purpose, to enable easy "hijacking". OTOH when I checked the arguments in gdb they looked always sane, up to the last and crashing realloc() call. That's why I do not expect seeing anything unusual there. Valgrind did not see any bad free()s either. > there can be some call abi issue (register clobbering, > stack alignment,..) because of the asm, but that's hard > to check. Is musl in any way special compared to glibc/uclibc in its register usage? > you may try tracing malloc calls (i don't know an easy > way other than instrumenting musl, you can try python > scripting gdb, the default gdb command language is not > enough for reporting malloc args and return values). This is something I wished to avoid. It does not promise much either, but I may possibly try this if nothing else helps. Thanks everyone for the help! Rune
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