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[PATCH bpf-next v3 2/3] bpf, x86: clean garbage value in the stack of trampoline

menglong8.dong@gmail.com posted 3 patches 2 years, 8 months ago
There is a newer version of this series
[PATCH bpf-next v3 2/3] bpf, x86: clean garbage value in the stack of trampoline
Posted by menglong8.dong@gmail.com 2 years, 8 months ago 
From: Menglong Dong <imagedong@tencent.com>

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@tencent.com>
Signed-off-by: Menglong Dong <imagedong@tencent.com>
---
 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));
 		} 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));
 		}
 
@@ -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
Re: [PATCH bpf-next v3 2/3] bpf, x86: clean garbage value in the stack of trampoline
Posted by Alexei Starovoitov 2 years, 8 months ago 
On Wed, Jun 07, 2023 at 08:59:10PM +0800, menglong8.dong@gmail.com wrote:
> From: Menglong Dong <imagedong@tencent.com>
> 
> 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@tencent.com>
> Signed-off-by: Menglong Dong <imagedong@tencent.com>
> ---
>  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.

>  		}
>  
> @@ -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
>
Re: [PATCH bpf-next v3 2/3] bpf, x86: clean garbage value in the stack of trampoline
Posted by Menglong Dong 2 years, 8 months ago 
On Thu, Jun 8, 2023 at 4:03 AM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Wed, Jun 07, 2023 at 08:59:10PM +0800, menglong8.dong@gmail.com wrote:
> > From: Menglong Dong <imagedong@tencent.com>
> >
> > 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@tencent.com>
> > Signed-off-by: Menglong Dong <imagedong@tencent.com>
> > ---
> >  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
> >

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