Re: Bringing rseq back into glibc

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Nov 18, 2021, at 1:48 PM, Cristian Rodríguez crrodriguez@opensuse.org wrote:

> On Thu, Nov 18, 2021 at 7:17 AM Florian Weimer via Libc-alpha
> <libc-alpha@sourceware.org> wrote:
> 
>> 4. Add public symbols __rseq_abi_offset, __rseq_abi_size (currently 32
>>    or 0), __rseq_abi_flags (currently 0).  __rseq_abi_offset is the
>>    offset to add to the thread pointer (see __builtin_thread_pointer) to
>>    get to the rseq area.  They will be public ABI symbols.  These
>>    variables are initialized before user code runs, and changing the
>>    results in undefined behavior.
> 
> Why not then __get_rseq_whatwever functions and not variables ? or
> maybe writing to these variables results in a compiler or linker error
> instead of UB ?

rseq critical sections cannot issue function calls, and also function calls
are noticeably expensive compared to an rseq critical section. So all users
would end up needing to make a local copy of the information fetched by those
getters.

So rather than require all those extra per-user copies, I suspect exposing
a single copy through public glibc symbols is more efficient.

The downside is indeed that writing to those variables is UB.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com