On Thu, Jun 03, 2021 at 05:51:34PM +0100, Mark Brown wrote:

On Thu, Jun 03, 2021 at 04:40:35PM +0100, Dave Martin wrote:

quoted

Do we know how libcs will detect that they don't need to do the
mprotect() calls?  Do we need a detection mechanism at all?

Ignoring certain errors from mprotect() when ld.so is trying to set
PROT_BTI on the main executable's code pages is probably a reasonable,
backwards-compatible compromise here, but it seems a bit wasteful.

I think the theory was that they would just do the mprotect() calls and
ignore any errors as they currently do, or declare that they depend on a
new enough kernel version I guess (not an option for glibc but might be
for others which didn't do BTI yet).

I think we discussed the possibility of an AT_FLAGS bit. Until recently,
this field was 0 but it gained a new bit now. If we are to expose this
to arch-specific things, it may need some reservations. Anyway, that's
an optimisation that can be added subsequently.

-- 
Catalin

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