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Date:   Thu, 16 Jul 2020 22:36:07 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Maciej Fijalkowski <maciej.fijalkowski@...el.com>, ast@...nel.org
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org, bjorn.topel@...el.com,
        magnus.karlsson@...el.com
Subject: Re: [PATCH bpf-next 1/5] bpf, x64: use %rcx instead of %rax for tail
 call retpolines

On 7/16/20 1:36 AM, Maciej Fijalkowski wrote:
> Currently, %rax is used to store the jump target when BPF program is
> emitting the retpoline instructions that are handling the indirect
> tailcall.
> 
> There is a plan to use %rax for different purpose, which is storing the
> tail call counter. In order to preserve this value across the tailcalls,
> use %rcx instead for jump target storage in retpoline instructions.
> 
> Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
> ---
>   arch/x86/include/asm/nospec-branch.h | 16 ++++++++--------
>   1 file changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> index e7752b4038ff..e491c3d9f227 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -314,19 +314,19 @@ static inline void mds_idle_clear_cpu_buffers(void)
>    *    lfence
>    *    jmp spec_trap
>    *  do_rop:
> - *    mov %rax,(%rsp) for x86_64
> + *    mov %rcx,(%rsp) for x86_64
>    *    mov %edx,(%esp) for x86_32
>    *    retq
>    *
>    * Without retpolines configured:
>    *
> - *    jmp *%rax for x86_64
> + *    jmp *%rcx for x86_64
>    *    jmp *%edx for x86_32
>    */
>   #ifdef CONFIG_RETPOLINE
>   # ifdef CONFIG_X86_64
> -#  define RETPOLINE_RAX_BPF_JIT_SIZE	17
> -#  define RETPOLINE_RAX_BPF_JIT()				\
> +#  define RETPOLINE_RCX_BPF_JIT_SIZE	17
> +#  define RETPOLINE_RCX_BPF_JIT()				\
>   do {								\
>   	EMIT1_off32(0xE8, 7);	 /* callq do_rop */		\
>   	/* spec_trap: */					\
> @@ -334,7 +334,7 @@ do {								\
>   	EMIT3(0x0F, 0xAE, 0xE8); /* lfence */			\
>   	EMIT2(0xEB, 0xF9);       /* jmp spec_trap */		\
>   	/* do_rop: */						\
> -	EMIT4(0x48, 0x89, 0x04, 0x24); /* mov %rax,(%rsp) */	\
> +	EMIT4(0x48, 0x89, 0x0C, 0x24); /* mov %rcx,(%rsp) */	\
>   	EMIT1(0xC3);             /* retq */			\
>   } while (0)
>   # else /* !CONFIG_X86_64 */
> @@ -352,9 +352,9 @@ do {								\
>   # endif
>   #else /* !CONFIG_RETPOLINE */
>   # ifdef CONFIG_X86_64
> -#  define RETPOLINE_RAX_BPF_JIT_SIZE	2
> -#  define RETPOLINE_RAX_BPF_JIT()				\
> -	EMIT2(0xFF, 0xE0);       /* jmp *%rax */
> +#  define RETPOLINE_RCX_BPF_JIT_SIZE	2
> +#  define RETPOLINE_RCX_BPF_JIT()				\
> +	EMIT2(0xFF, 0xE1);       /* jmp *%rcx */

Hmm, so the target prog gets loaded into rax in emit_bpf_tail_call_indirect()
but then you jump into rcx? What am I missing? This still needs to be bisectable.

>   # else /* !CONFIG_X86_64 */
>   #  define RETPOLINE_EDX_BPF_JIT()				\
>   	EMIT2(0xFF, 0xE2)        /* jmp *%edx */
> 

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