Re: Invalid program counters and unwinding

[Jeff Law <law@redhat.com>]

On 06/26/2018 03:26 AM, Florian Weimer wrote:
> I'm looking at ways to speed up _Unwind_Find_FDE when libgcc is running
> on top of glibc.  I have something (at the design level, with some of
> the code written) which allows me to get a pointer to the
> PT_GNU_EH_FRAME segment in memory in a lock-free fashion (so it would
> also be async-signal safe).
> 
> This part works also when the program counter used in the search is
> invalid and does not point to within a loaded object, even in the case
> of concurrent dlopen/dlclose.
> 
> However, it's still necessary to read the PT_GNU_EH_FRAME data itself,
> and if _Unwind_Find_FDE is not a valid program counter found on the
> stack (with in a caller, where unmapping it with dlclose would be
> invalid), it could happen that it is a random address in *another*,
> unrelated object, which then gets dlclose'd (which is valid).
> 
> The current glibc-based implementation in libgcc calls dl_iterate_phdr,
> which acquires a lock blocking dlclose for the entire duration of the
> iteration.  But I think this still doesn't support arbitrary, random PC
> values because in the worst case, the PC value looks valid, we find some
> unrelated FDE data with an associated personality routine, and end up
> calling that, with disastrous consequences.
> 
> So it looks to me that the caller of _Unwind_Find_FDE needs to ensure
> that the PC is a valid element of the call stack.  Is this a correct
> assumption?
> 
> I have some ideas how make reading the PT_GNU_EH_FRAME data safe, but
> the question is whether we actually need that.
> 
> Previous discussions:
> 
> https://gcc.gnu.org/ml/gcc/2013-05/msg00253.html
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71744
> https://sourceware.org/ml/libc-alpha/2016-07/msg00613.html
>   (patch with a spread lock, still not async-signal-safe)
You might also want to look at RH BZ 1293594 which I think has pointers
back to an issue from 2008 :(

Jeff