Re: [RFC PATCH] glibc: Perform rseq(2) registration at nptl init and thread creation

On Thu, 20 Sep 2018, Mathieu Desnoyers wrote:

Are you saying glibc has an explicit check for the kernel version visible
from /proc before using specific features ? If so, how can this work with
the variety of feature backports we find in the distribution kernels out
there ?

See sysdeps/unix/sysv/linux/dl-sysdep.c and 
sysdeps/unix/sysv/linux/dl-osinfo.h.  As I said, Carlos has proposed 
removing that check.

For too-old headers at compile time, one possibility is that we don't event
expose the __rseq_abi TLS symbol. OTOH, if we need to keep exposing it anyway
for ABI consistency purposes, then we'd leave its cpu_id field at the initial
value (-1). But that would require that we copy linux/rseq.h into the glibc
source tree.

The ABI needs to be independent of the kernel headers used.  I don't think 
you need to copy linux/rseq.h; all you should need is to e.g. define an 
array of suitable size and alignment with the relevant member initialized 
and a suitable explanatory comment.

-- 
Joseph S. Myers
joseph@codesourcery.com