Re: [PATCH V5 03/50] x86/traps: Remove stack-protector from traps.c
From: Peter Zijlstra
Date: Thu Nov 18 2021 - 14:55:25 EST
On Wed, Nov 10, 2021 at 07:56:49PM +0800, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> When stack-protector is enabled, the compiler adds some instrument code
> at the beginning and the end of some functions. Many functions in traps.c
> are non-instrumentable. Moreover, stack-protector code in the beginning
> of the affected function accesses the canary that might be watched by
> hardware breakpoints which also violate the non-instrumentable
> nature of some functions and might cause infinite recursive #DB because
> the canary is accessed before resetting the dr7.
> So it is better to remove stack-protector from traps.c.
> It is also prepared for later patches that move some entry code into
> traps.c, some of which can NOT use percpu register until gsbase is
> properly switched. And stack-protector depends on the percpu register
> to work.
> Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> arch/x86/kernel/Makefile | 1 +
> 1 file changed, 1 insertion(+)
> diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
> index 2ff3e600f426..8ac45801ba8b 100644
> --- a/arch/x86/kernel/Makefile
> +++ b/arch/x86/kernel/Makefile
> @@ -50,6 +50,7 @@ KCOV_INSTRUMENT := n
> CFLAGS_head$(BITS).o += -fno-stack-protector
> CFLAGS_cc_platform.o += -fno-stack-protector
> +CFLAGS_traps.o += -fno-stack-protector
Well, there's a lot more noinstr than just in traps. There's also real C
code in traps. This isn't really a solution.
I think GCC has recently grown __attribute__((no_stack_protector)),
which should be added to noinstr (GCC-11 and above).
Additionally we could add code to objtool to detect this problem.