* Topi Miettinen:

quoted

The dynamic loader has to process the LOAD segments to get to the ELF
note that says to enable BTI.  Maybe we could do a first pass and
load only the segments that cover notes.  But that requires lots of
changes to generic code in the loader.

What if the loader always enabled BTI for PROT_EXEC pages, but then
when discovering that this was a mistake, mprotect() the pages without
BTI?

Is that architecturally supported?  How costly is the mprotect change if
the pages have not been faulted in yet?

Then both BTI and MDWX would work and the penalty of not getting
MDWX would fall to non-BTI programs. What's the expected proportion of
BTI enabled code vs. disabled in the future, is it perhaps expected
that a distro would enable the flag globally so eventually only a few
legacy programs might be unprotected?

Eventually, I expect that mainstream distributions build everything for
BTI, so yes, the PROT_BTI removal would only be needed for legacy
programs.

Thanks,
Florian
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