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

[Greg KH <greg@kroah.com>]

On Wed, Sep 01, 2021 at 01:30:37PM +0200, Greg KH wrote:
> 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.

All _4_ patches I mean.  now dropped...