Re: [RFC PATCH glibc 1/4] glibc: Perform rseq(2) registration at C startup and thread creation (v6)

[Mathieu Desnoyers]

----- On Jan 30, 2019, at 4:10 PM, Joseph Myers joseph@xxxxxxxxxxxxxxxx wrote:

> On Wed, 30 Jan 2019, Mathieu Desnoyers wrote:
> 
>> #if defined (__NR_rseq) && !defined (RSEQ_SIG)
>> # error "UAPI headers support rseq system call, but glibc does not define
>> RSEQ_SIG."
>> #endif
>> 
>> Would that take care of your concerns ?
> 
> That would of course need appropriate conditionals based on the most
> recent kernel version for which a given glibc version has been updated, so
> that using new kernel headers with an existing glibc release does not make
> the build fail (cf. the test of syscall-names.list).

The test I hint at above would not be for the glibc build per se. It would
be for a check that glibc implements support for all the system calls
available in the kernel headers (if such a test target currently exists).

> And being able to
> write such a test only solves one half of the problem - it needs to be
> easy to determine what value to put in that header in glibc for an
> architecture that's newly gained support in the kernel, *without* needing
> any architecture expertise.

I'm afraid this requirement is incompatible with the nature of the RSEQ
signature. This signature may be required to be a specific trap instruction
by the architecture, so deciding on its value without architecture expertise
is not possible.

Thanks,

Mathieu

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