Re: [PATCH bpf-next v5 1/3] bpf, x86: clean garbage values when store args from regs into stack

[Menglong Dong]

On Mon, Jun 19, 2023 at 6:52 AM Yonghong Song <yhs@xxxxxxxx> wrote:
>
>
>
> On 6/12/23 7:52 PM, menglong8.dong@xxxxxxxxx wrote:
> > From: Menglong Dong <imagedong@xxxxxxxxxxx>
> >
> > There are garbage values in upper bytes when we store the arguments
> > into stack in save_regs() if the size of the argument less then 8.
> >
> > As we already reserve 8 byte for the arguments in regs and stack,
> > it is ok to store/restore the regs in BPF_DW size. Then, the garbage
> > values in upper bytes will be cleaned.
>
> Please make it clear that there are no bugs in the existing code
> since for each argument, a type case will happen like
>      (parameter_type)ctx[stack_slot]
> where ctx[] is an u64 type array. The compiler will generate
> correct code to do type casting so garbage value will not impact
> the final result. But indeed, uniformly
> do save/restore with BPF_DW size can simplify code.
>

Yeah, this is to prepare for the next commit and no bugs
in the existing code. I'll make it clear in the commit log in
the next version.

Thanks!
Menglong Dong

> >
> > Signed-off-by: Menglong Dong <imagedong@xxxxxxxxxxx>
>
> Acked-by: Yonghong Song <yhs@xxxxxx>
>
> > ---
> >   arch/x86/net/bpf_jit_comp.c | 35 ++++++-----------------------------
> >   1 file changed, 6 insertions(+), 29 deletions(-)
> >
> > diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> > index 1056bbf55b17..a407fbbffecd 100644
> > --- a/arch/x86/net/bpf_jit_comp.c
> > +++ b/arch/x86/net/bpf_jit_comp.c
> > @@ -1860,57 +1860,34 @@ st:                   if (is_imm8(insn->off))
> >   static void save_regs(const struct btf_func_model *m, u8 **prog, int nr_regs,
> >                     int stack_size)
> >   {
> > -     int i, j, arg_size;
> > -     bool next_same_struct = false;
> > +     int i;
> >
> >       /* Store function arguments to stack.
> >        * For a function that accepts two pointers the sequence will be:
> >        * mov QWORD PTR [rbp-0x10],rdi
> >        * mov QWORD PTR [rbp-0x8],rsi
> >        */
> > -     for (i = 0, j = 0; i < min(nr_regs, 6); i++) {
> > -             /* The arg_size is at most 16 bytes, enforced by the verifier. */
> > -             arg_size = m->arg_size[j];
> > -             if (arg_size > 8) {
> > -                     arg_size = 8;
> > -                     next_same_struct = !next_same_struct;
> > -             }
> > -
> > -             emit_stx(prog, bytes_to_bpf_size(arg_size),
> > -                      BPF_REG_FP,
> > +     for (i = 0; i < min(nr_regs, 6); i++)
> > +             emit_stx(prog, BPF_DW, BPF_REG_FP,
> >                        i == 5 ? X86_REG_R9 : BPF_REG_1 + i,
> >                        -(stack_size - i * 8));
> > -
> > -             j = next_same_struct ? j : j + 1;
> > -     }
> >   }
> >
> [...]