Re: [PATCH 4.14 1/4] bpf: Do not use ax register in interpreter on div/mod

[Greg KH <greg@kroah.com>]

On Mon, Aug 30, 2021 at 03:32:08PM -0300, Thadeu Lima de Souza Cascardo wrote:
> From: Daniel Borkmann <daniel@iogearbox.net>
> 
> Partially undo old commit 144cd91c4c2b ("bpf: move tmp variable into ax
> register in interpreter"). The reason we need this here is because ax
> register will be used for holding temporary state for div/mod instruction
> which otherwise interpreter would corrupt. This will cause a small +8 byte
> stack increase for interpreter, but with the gain that we can use it from
> verifier rewrites as scratch register.
> 
> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
> Reviewed-by: John Fastabend <john.fastabend@gmail.com>
> [cascardo: This partial revert is needed in order to support using AX for
> the following two commits, as there is no JMP32 on 4.19.y]
> Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
> ---
>  kernel/bpf/core.c | 32 +++++++++++++++-----------------
>  1 file changed, 15 insertions(+), 17 deletions(-)
> 
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index e7211b0fa27c..30d871be9974 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -616,9 +616,6 @@ static int bpf_jit_blind_insn(const struct bpf_insn *from,
>  	 * below.
>  	 *
>  	 * Constant blinding is only used by JITs, not in the interpreter.
> -	 * The interpreter uses AX in some occasions as a local temporary
> -	 * register e.g. in DIV or MOD instructions.
> -	 *
>  	 * In restricted circumstances, the verifier can also use the AX
>  	 * register for rewrites as long as they do not interfere with
>  	 * the above cases!
> @@ -951,6 +948,7 @@ static unsigned int ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn,
>  	u32 tail_call_cnt = 0;
>  	void *ptr;
>  	int off;
> +	u64 tmp;
>  
>  #define CONT	 ({ insn++; goto select_insn; })
>  #define CONT_JMP ({ insn++; goto select_insn; })
> @@ -1013,22 +1011,22 @@ static unsigned int ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn,
>  	ALU64_MOD_X:
>  		if (unlikely(SRC == 0))
>  			return 0;
> -		div64_u64_rem(DST, SRC, &AX);
> -		DST = AX;
> +		div64_u64_rem(DST, SRC, &tmp);
> +		DST = tmp;
>  		CONT;
>  	ALU_MOD_X:
>  		if (unlikely((u32)SRC == 0))
>  			return 0;
> -		AX = (u32) DST;
> -		DST = do_div(AX, (u32) SRC);
> +		tmp = (u32) DST;
> +		DST = do_div(tmp, (u32) SRC);
>  		CONT;
>  	ALU64_MOD_K:
> -		div64_u64_rem(DST, IMM, &AX);
> -		DST = AX;
> +		div64_u64_rem(DST, IMM, &tmp);
> +		DST = tmp;
>  		CONT;
>  	ALU_MOD_K:
> -		AX = (u32) DST;
> -		DST = do_div(AX, (u32) IMM);
> +		tmp = (u32) DST;
> +		DST = do_div(tmp, (u32) IMM);
>  		CONT;
>  	ALU64_DIV_X:
>  		if (unlikely(SRC == 0))
> @@ -1038,17 +1036,17 @@ static unsigned int ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn,
>  	ALU_DIV_X:
>  		if (unlikely((u32)SRC == 0))
>  			return 0;
> -		AX = (u32) DST;
> -		do_div(AX, (u32) SRC);
> -		DST = (u32) AX;
> +		tmp = (u32) DST;
> +		do_div(tmp, (u32) SRC);
> +		DST = (u32) tmp;
>  		CONT;
>  	ALU64_DIV_K:
>  		DST = div64_u64(DST, IMM);
>  		CONT;
>  	ALU_DIV_K:
> -		AX = (u32) DST;
> -		do_div(AX, (u32) IMM);
> -		DST = (u32) AX;
> +		tmp = (u32) DST;
> +		do_div(tmp, (u32) IMM);
> +		DST = (u32) tmp;
>  		CONT;
>  	ALU_END_TO_BE:
>  		switch (IMM) {
> -- 
> 2.30.2
> 

Oops, no, this patch causes build errors:

kernel/bpf/core.c: In function ‘___bpf_prog_run’:
kernel/bpf/core.c:951:13: error: redeclaration of ‘tmp’ with no linkage
  951 |         u64 tmp;
      |             ^~~
kernel/bpf/core.c:839:13: note: previous declaration of ‘tmp’ with type ‘u64’ {aka ‘long long unsigned int’}
  839 |         u64 tmp;
      |             ^~~
make[2]: *** [scripts/Makefile.build:329: kernel/bpf/core.o] Error 1


Please fix up and resend the whole series, as I will go drop these 3
patches from the 4.14.y queue now.

greg k-h