On Thu, Jun 8, 2023 at 4:03 AM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Wed, Jun 07, 2023 at 08:59:10PM +0800, 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.
> >
> > Reviewed-by: Jiang Biao <benbjiang@xxxxxxxxxxx>
> > Signed-off-by: Menglong Dong <imagedong@xxxxxxxxxxx>
> > ---
> > arch/x86/net/bpf_jit_comp.c | 19 ++++++-------------
> > 1 file changed, 6 insertions(+), 13 deletions(-)
> >
> > diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> > index 413b986b5afd..e9bc0b50656b 100644
> > --- a/arch/x86/net/bpf_jit_comp.c
> > +++ b/arch/x86/net/bpf_jit_comp.c
> > @@ -1878,20 +1878,16 @@ 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));
>
> This is ok,
>
> > } else {
> > /* copy function arguments from origin stack frame
> > * into current stack frame.
> > */
> > - 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));
>
> But this is not.
> See https://godbolt.org/z/qW17f6cYe
> mov dword ptr [rsp], 6
>
> the compiler will store 32-bit only. The upper 32-bit are still garbage.
Enn......I didn't expect this case, and it seems
that this only happens on clang. With gcc,
"push 6" is used.
I haven't found a solution for this case, and it seems
not worth it to add an extra insn to clean the garbage
values.
Does anyone have any ideas here?
Thanks!
Menglong Dong
>
> > }
> >
> > @@ -1918,7 +1914,7 @@ 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));
> > @@ -1949,12 +1945,9 @@ 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));
> > }
> >
> > --
> > 2.40.1
> >
