Date: Fri, 24 Jul 2015 12:53:10 +0200 From: Szabolcs Nagy <nsz@...t70.net> To: musl@...ts.openwall.com Subject: Re: [PATCH] fix atexit when it is called from an atexit handler * Jens Gustedt <jens.gustedt@...ia.fr> [2015-07-24 08:52:23 +0200]: > Am Freitag, den 24.07.2015, 02:16 +0200 schrieb Szabolcs Nagy: > > > > $ printf ' > > set breakpoint pending on > > break __cxa_atexit > > commands > > frame 0 > > continue > > end > > run > > ' |gdb clang 2>/dev/null |grep '^Breakpoint' |wc -l > > 610 > > interesting > > > i.e. clang registers 610 atexit handlers. > > hm, I don't think these are atexit handlers proper. I looked at the > addresses (sort -u | less), most of them are different, so they are probably > direct calls to __cxa_atexit, not to atexit. they are destructors for static objects, but those are atexit handlers too: they get into the same list. (the c++ standard does not guarantee limits for static objects with destructors even though it specifies the limit for atexit handlers and requires interleaved execution of atexit handlers and destructors, so the only reasonable implementation is to use the same mechanism for them. in theory we could have a fixed array of 32 list items for c atexit handlers and always malloc items for c++, but that seems extra work just to provide the minimal possible guarantees of the standards.) > > i don't find atexit after exit dangerous: it is > > a programmer error if there are a lot of atexit > > calls, > > that's what I meant > > > not an input dependent dos attack surface. > > I am not an expert in these, but AFAIR all function pointers that are > stored in predictable places are attack surfaces. if there is memory corruption and an attacker controls the pointers then it is an attack surface, but without memory corruption the number of atexit handlers registered rarely depends on input at runtime, it's a design decision by the programmer.
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