i386: Lazy binding trampoline and vector register usage

[Florian Weimer <fweimer@redhat.com>]

We have this in sysdeps/i386/Makefile:

# Make sure no code in ld.so uses mm/xmm/ymm/zmm registers on i386 since
# the first 3 mm/xmm/ymm/zmm registers are used to pass vector parameters
# which must be preserved.
# With SSE disabled, ensure -fpmath is not set to use sse either.
rtld-CFLAGS += -mno-sse -mno-mmx -mfpmath=387
ifeq ($(subdir),elf)
CFLAGS-.os += $(if $(filter $(@F),$(patsubst %,%.os,$(all-rtld-routines))),\
		   $(rtld-CFLAGS))

tests-special += $(objpfx)tst-ld-sse-use.out
$(objpfx)tst-ld-sse-use.out: ../sysdeps/i386/tst-ld-sse-use.sh $(objpfx)ld.so
	@echo "Checking ld.so for SSE register use.  This will take a few seconds..."
	$(BASH) $< $(objpfx) '$(NM)' '$(OBJDUMP)' '$(READELF)' > $@; \
	$(evaluate-test)
else
CFLAGS-.os += $(if $(filter rtld-%.os,$(@F)), $(rtld-CFLAGS))
endif

The idea is that we do not need to save and restore vector registers in
the trampoline (or align the stack) if we compile ld.so in such a way
that only general registers are used.  But that does not actually work
in all cases because lazy binding can call malloc, which lives in
libc.so or might even be interposed, and is thus free to use vector
registers.

What should we do about this?  Calling malloc from _dl_fixup is unsafe
for other reasons because lazy binding can happen in signal handlers, so
maybe this would be fixed if we switched to a non-interposable
async-signal-safe allocator?

(I found this by code inspection.  I have not seen actual crashes/wrong
results at run time.)

Thanks,
Florian