Re: [PATCH bpf-next] bpf, x86: Simplify the parsing logic of structure parameters

[Pu Lehui]

On 2023/1/5 2:24, Yonghong Song wrote:
> On 1/2/23 5:31 PM, Pu Lehui wrote:
> > From: Pu Lehui <pulehui@xxxxxxxxxx>
> >
> > Extra_nregs of structure parameters and nr_args can be
> > added directly at the beginning, and using a flip flag
> > to identifiy structure parameters. Meantime, renaming
> > some variables to make them more sense.
> >
> > Signed-off-by: Pu Lehui <pulehui@xxxxxxxxxx>
>
> Thanks for refactoring. Using nr_regs instead of nr_args indeed
> making things easier to understand. Ack with a few nits below.
>
> Acked-by: Yonghong Song <yhs@xxxxxx>
>
> > ---
> >   arch/x86/net/bpf_jit_comp.c | 99 +++++++++++++++++--------------------
> >   1 file changed, 46 insertions(+), 53 deletions(-)
> >
> > diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> > index e3e2b57e4e13..e7b72299f5a4 100644
> > --- a/arch/x86/net/bpf_jit_comp.c
> > +++ b/arch/x86/net/bpf_jit_comp.c
> > @@ -1839,62 +1839,57 @@ st:            if (is_imm8(insn->off))
> >       return proglen;
> >   }
> > -static void save_regs(const struct btf_func_model *m, u8 **prog, int 
> > nr_args,
> > +static void save_regs(const struct btf_func_model *m, u8 **prog, int 
> > nr_regs,
> >                 int stack_size)
> >   {
> > -    int i, j, arg_size, nr_regs;
> > +    int i, j, arg_size;
> > +    bool is_struct = false;
> > +
> >       /* 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_args, 6); i++) {
> > -        if (m->arg_flags[i] & BTF_FMODEL_STRUCT_ARG) {
> > -            nr_regs = (m->arg_size[i] + 7) / 8;
> > +    for (i = 0, j = 0; i < min(nr_regs, 6); i++) {
> > +        arg_size = m->arg_size[j];
> > +        if (arg_size > 8) {
> >               arg_size = 8;
> > -        } else {
> > -            nr_regs = 1;
> > -            arg_size = m->arg_size[i];
> > +            is_struct ^= 1;
> >           }
> > -        while (nr_regs) {
> > -            emit_stx(prog, bytes_to_bpf_size(arg_size),
> > -                 BPF_REG_FP,
> > -                 j == 5 ? X86_REG_R9 : BPF_REG_1 + j,
> > -                 -(stack_size - j * 8));
> > -            nr_regs--;
> > -            j++;
> > -        }
> > +        emit_stx(prog, bytes_to_bpf_size(arg_size),
> > +             BPF_REG_FP,
> > +             i == 5 ? X86_REG_R9 : BPF_REG_1 + i,
> > +             -(stack_size - i * 8));
> > +
> > +        j = is_struct ? j : j + 1;
> >       }
> >   }
> > -static void restore_regs(const struct btf_func_model *m, u8 **prog, 
> > int nr_args,
> > +static void restore_regs(const struct btf_func_model *m, u8 **prog, 
> > int nr_regs,
> >                int stack_size)
> >   {
> > -    int i, j, arg_size, nr_regs;
> > +    int i, j, arg_size;
> > +    bool is_struct = false;
>
> Maybe
>      bool next_same_struct = false
> to better characterize what it means?

agree, will do as suggested bellow.

> >       /* Restore function arguments from stack.
> >        * For a function that accepts two pointers the sequence will be:
> >        * EMIT4(0x48, 0x8B, 0x7D, 0xF0); mov rdi,QWORD PTR [rbp-0x10]
> >        * EMIT4(0x48, 0x8B, 0x75, 0xF8); mov rsi,QWORD PTR [rbp-0x8]
> >        */
> > -    for (i = 0, j = 0; i < min(nr_args, 6); i++) {
> > -        if (m->arg_flags[i] & BTF_FMODEL_STRUCT_ARG) {
> > -            nr_regs = (m->arg_size[i] + 7) / 8;
> > +    for (i = 0, j = 0; i < min(nr_regs, 6); i++) {
>
> Let us put a comment here so the later users can understand the logic
> behind 'is_struct ^= 1'.
>
> /* 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;
> > -        } else {
> > -            nr_regs = 1;
> > -            arg_size = m->arg_size[i];
> > +            is_struct ^= 1;
>
> next_same_struct = !next_same_struct;
>
> The same for above save_regs().
>
> >           }
> > -        while (nr_regs) {
> > -            emit_ldx(prog, bytes_to_bpf_size(arg_size),
> > -                 j == 5 ? X86_REG_R9 : BPF_REG_1 + j,
> > -                 BPF_REG_FP,
> > -                 -(stack_size - j * 8));
> > -            nr_regs--;
> > -            j++;
> > -        }
> > +        emit_ldx(prog, bytes_to_bpf_size(arg_size),
> > +             i == 5 ? X86_REG_R9 : BPF_REG_1 + i,
> > +             BPF_REG_FP,
> > +             -(stack_size - i * 8));
> > +
> > +        j = is_struct ? j : j + 1;
> >       }
> >   }
> [...]