Re: [PATCH bpf-next v4 2/3] bpf, x86: clean garbage value in the stack of trampoline

[Menglong Dong]

On Sat, Jun 10, 2023 at 11:20 AM Yonghong Song <yhs@xxxxxxxx> wrote:
>
>
>
> On 6/9/23 2:56 AM, 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.
> >
> > Signed-off-by: Menglong Dong <imagedong@xxxxxxxxxxx>
> > ---
> > v4:
> > - clean grabage value when argument count is 7
> > ---
> >   arch/x86/net/bpf_jit_comp.c | 45 ++++++++++++++++++++++++++-----------
> >   1 file changed, 32 insertions(+), 13 deletions(-)
> >
> > diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> > index a767e13c8c85..f6f51a5d14db 100644
> > --- a/arch/x86/net/bpf_jit_comp.c
> > +++ b/arch/x86/net/bpf_jit_comp.c
> > @@ -1857,6 +1857,28 @@ st:                    if (is_imm8(insn->off))
> >       return proglen;
> >   }
> >
> > +static inline void clean_garbage(u8 **pprog, int nr_regs, int stack_size,
> > +                              int arg_size)
> > +{
> > +     u8 *prog;
> > +
> > +     /* clean potential garbage values in upper 32-bit. 'stack_size'
> > +      * here is the offset of the 7th argument on-stack.
>
> Here, we have a huge assumption that if only 7 registers, compiler might
> just allocate a 8-byte stack space and write the value to it. If the
> type of the value is <= 32bit, a 32bit store will be on the stack.
> So in this case, the upper 32bit needs to be cleared.
> If the number of arguments (in terms of number of registers) is more
> than 7, the extra arguments will be 'pushed' to the stack, so there
> is no garbage. This could be true. But please add enough details
> here so people knows why we special case this particular instance.
>

Yeah, this indeed is a huge assumption. I'll add more
comments here to help others understand this case.

Thanks!
Menglong Dong

> > +      */
> > +     if (nr_regs == 7 && arg_size <= 4) {
> > +             int off = -(stack_size - 4);
> > +
> > +             prog = *pprog;
> > +             /* mov DWORD PTR [rbp + off], 0 */
> > +             if (!is_imm8(off))
> > +                     EMIT2_off32(0xC7, 0x85, off);
> > +             else
> > +                     EMIT3(0xC7, 0x45, off);
> > +             EMIT(0, 4);
> > +             *pprog = prog;
> > +     }
> > +}
> > +
> >   static void save_regs(const struct btf_func_model *m, u8 **prog, int nr_regs,
> >                     int stack_size)
> >   {
> > @@ -1878,8 +1900,7 @@ static void save_regs(const struct btf_func_model *m, u8 **prog, int nr_regs,
> >
> >               if (i <= 5) {
> >                       /* copy function arguments from regs into stack */
> > -                     emit_stx(prog, bytes_to_bpf_size(arg_size),
> > -                              BPF_REG_FP,
> > +                     emit_stx(prog, BPF_DW, BPF_REG_FP,
> >                                i == 5 ? X86_REG_R9 : BPF_REG_1 + i,
> >                                -(stack_size - i * 8));
> >               } else {
> > @@ -1893,17 +1914,16 @@ static void save_regs(const struct btf_func_model *m, u8 **prog, int nr_regs,
> >                        *   8(return addr of the caller)
> >                        * which means: rbp + 24
> >                        */
> > -                     emit_ldx(prog, bytes_to_bpf_size(arg_size),
> > -                              BPF_REG_0, BPF_REG_FP,
> > +                     emit_ldx(prog, BPF_DW, BPF_REG_0, BPF_REG_FP,
> >                                (i - 6) * 8 + 0x18);
> > -                     emit_stx(prog, bytes_to_bpf_size(arg_size),
> > -                              BPF_REG_FP,
> > -                              BPF_REG_0,
> > +                     emit_stx(prog, BPF_DW, BPF_REG_FP, BPF_REG_0,
> >                                -(stack_size - i * 8));
> >               }
> >
> >               j = next_same_struct ? j : j + 1;
> >       }
> > +
> > +     clean_garbage(prog, nr_regs, stack_size - 6 * 8, arg_size);
> >   }
> >
> >   static void restore_regs(const struct btf_func_model *m, u8 **prog, int nr_regs,
> > @@ -1925,7 +1945,7 @@ static void restore_regs(const struct btf_func_model *m, u8 **prog, int nr_regs,
> >                       next_same_struct = !next_same_struct;
> >               }
> >
> > -             emit_ldx(prog, bytes_to_bpf_size(arg_size),
> > +             emit_ldx(prog, BPF_DW,
> >                        i == 5 ? X86_REG_R9 : BPF_REG_1 + i,
> >                        BPF_REG_FP,
> >                        -(stack_size - i * 8));
> > @@ -1956,17 +1976,16 @@ static void prepare_origin_stack(const struct btf_func_model *m, u8 **prog,
> >               }
> >
> >               if (i > 5) {
> > -                     emit_ldx(prog, bytes_to_bpf_size(arg_size),
> > -                              BPF_REG_0, BPF_REG_FP,
> > +                     emit_ldx(prog, BPF_DW, BPF_REG_0, BPF_REG_FP,
> >                                (i - 6) * 8 + 0x18);
> > -                     emit_stx(prog, bytes_to_bpf_size(arg_size),
> > -                              BPF_REG_FP,
> > -                              BPF_REG_0,
> > +                     emit_stx(prog, BPF_DW, BPF_REG_FP, BPF_REG_0,
> >                                -(stack_size - (i - 6) * 8));
> >               }
> >
> >               j = next_same_struct ? j : j + 1;
> >       }
> > +
> > +     clean_garbage(prog, nr_regs, stack_size, arg_size);
> >   }
> >
> >   static int invoke_bpf_prog(const struct btf_func_model *m, u8 **pprog,