Re: [PATCH v2] x86/retpoline: Add clang support

[David Woodhouse]

On Wed, 2018-02-07 at 13:52 -0800, Guenter Roeck wrote:
> clang has its own set of compiler options for retpoline support.
> 
> Link: https://github.com/llvm-mirror/clang/commit/0d816739a82da29748caf88570affb9715e18b69
> Link: https://github.com/llvm-mirror/llvm/commit/fd5a8723ce9f2a6b250e85972ef859e4253ea95d
> Link: https://github.com/llvm-mirror/llvm/commit/59b64490fda69d29bb42cfdf7eec37bcc31ff833
> Cc: David Woodhouse <dwmw@xxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: gnomes@xxxxxxxxxxxxxxxxxxx
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Cc: thomas.lendacky@xxxxxxx
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Jiri Kosina <jikos@xxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxx>
> Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxxx>
> Cc: Paul Turner <pjt@xxxxxxxxxx>
> Signed-off-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> ---
> v2: llvm has been updated to use the same thunk names as gcc.
> ÂÂÂÂTested with:
> ÂÂÂÂclang version 7.0.0
> 	(https://git.llvm.org/git/clang.git/
> 		848874aed95a913fb45f363120500cebfe54e2ef)
> 	(https://git.llvm.org/git/llvm.git/
> 		3afd566557f3616881505db0d69f5d19bf55ae14)
> ÂÂÂÂcross-checked with gcc 7.3.0 (x86_64-linux-gcc.br_real (Buildroot
> ÂÂÂÂ2018.02-rc1) 7.3.0).
> 
> Tested with 64-bit builds only; 32-bit images fail to build with clang
> with various unrelated errors and are difficult to test.
> 
> I had to change '+=' to '=' below since make otherwise sets
> RETPOLINE_CFLAGS to " ", and the subsequent ifneq would always match.
> This is also the reason for the "ifeq ($(RETPOLINE_CFLAGS),)".
> If there is another/different/better way to handle this, please let
> me know.

See
http://git.infradead.org/users/dwmw2/linux-retpoline.git/commitdiff/82a1f41600


> There are curently lots of warnings when building an image with clang.
> 
> ./include/linux/init.h:134:6: warning:
> 	unknown attribute 'indirect_branch' ignored
> 
> I was inclined to add "&& !defined(__clang__)" to the condition for the
> __noretpoline define to fix the problem, but concluded that this should
> be a separate patch unless it can be addressed in clang.

... where I fixed that too as well as providing the
__x86_indirect_thunk for 32-bit builds (which are broken for other
reasons anyway).

We do still need the %V asm constraint modifier to work in clang
though, because otherwise it still doesn't build if you have any
hypervisor guest support.

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