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

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On Wed, Sep 01, 2021 at 01:31:18PM +0200, Greg KH wrote:
> 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@xxxxxxxxxxxxx>
> > > 
> > > 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@xxxxxxxxxxxxx>
> > > Reviewed-by: John Fastabend <john.fastabend@xxxxxxxxx>
> > > [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@xxxxxxxxxxxxx>
> > > ---
> > >  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...

Ah... it seems I only built it with CONFIG_BPF_JIT_ALWAYS_ON. I will build with
both that option on and off and check the results.

Thanks for catching this.

Cascardo.



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