On 11.11.2020 12:15, Roger Pau Monné wrote: > On Fri, Oct 23, 2020 at 10:38:04AM +0200, Jan Beulich wrote: >> Under certain conditions CPUs can speculate into the instruction stream >> past a RET instruction. Guard against this just like 3b7dab93f240 >> ("x86/spec-ctrl: Protect against CALL/JMP straight-line speculation") >> did - by inserting an "INT $3" insn. It's merely the mechanics of how to >> achieve this that differ: A set of macros gets introduced to post- >> process RET insns issued by the compiler (or living in assembly files). >> >> Unfortunately for clang this requires further features their built-in >> assembler doesn't support: We need to be able to override insn mnemonics >> produced by the compiler (which may be impossible, if internally >> assembly mnemonics never get generated), and we want to use \(text) >> escaping / quoting in the auxiliary macro. >> >> Signed-off-by: Jan Beulich <jbeul...@suse.com> >> Acked-by: Roger Pau Monné <roger....@citrix.com> >> --- >> TBD: Would be nice to avoid the additions in .init.text, but a query to >> the binutils folks regarding the ability to identify the section >> stuff is in (by Peter Zijlstra over a year ago: >> https://sourceware.org/pipermail/binutils/2019-July/107528.html) >> has been left without helpful replies. >> --- >> v3: Use .byte 0xc[23] instead of the nested macros. >> v2: Fix build with newer clang. Use int3 mnemonic. Also override retq. >> >> --- a/xen/Makefile >> +++ b/xen/Makefile >> @@ -145,7 +145,15 @@ t2 = $(call as-insn,$(CC) -I$(BASEDIR)/i >> # https://bugs.llvm.org/show_bug.cgi?id=36110 >> t3 = $(call as-insn,$(CC),".macro FOO;.endm"$(close); asm volatile >> $(open)".macro FOO;.endm",-no-integrated-as) >> >> -CLANG_FLAGS += $(call or,$(t1),$(t2),$(t3)) >> +# Check whether \(text) escaping in macro bodies is supported. >> +t4 = $(call as-insn,$(CC),".macro m ret:req; \\(ret) $$\\ret; .endm; m >> 8",,-no-integrated-as) >> + >> +# Check whether macros can override insn mnemonics in inline assembly. >> +t5 = $(call as-insn,$(CC),".macro ret; .error; .endm; .macro retq; .error; >> .endm",-no-integrated-as) > > I was going over this to post a bug report to LLVM, but it seems like > gcc also doesn't overwrite ret when using the above snippet: > > https://godbolt.org/z/oqsPTv
I can't see what's different from void test(void) { asm volatile (".macro ret; .error; .endm; .macro retq; .error; .endm"); } but this one produces "Error: .error directive invoked in source file" for me with both old and new gcc. Jan