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Re: i386: Lazy binding trampoline and vector register usage
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: Florian Weimer <fweimer at redhat dot com>
- Cc: GNU C Library <libc-alpha at sourceware dot org>
- Date: Wed, 8 Jan 2020 14:55:57 -0800
- Subject: Re: i386: Lazy binding trampoline and vector register usage
- References: <87lfra2as7.fsf@oldenburg2.str.redhat.com>
On Wed, Dec 18, 2019 at 2:22 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> 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?
Yes, we can do it for i386.
> (I found this by code inspection. I have not seen actual crashes/wrong
> results at run time.)
Thanks.
--
H.J.