Re: [PATCH bpf-next v2 2/5] bpf, x86: allow function arguments up to 14 for TRACING

From: Menglong Dong
Date: Mon Jun 05 2023 - 22:02:25 EST

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On Tue, Jun 6, 2023 at 4:11 AM Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
>
> On Fri, Jun 02, 2023 at 02:59:55PM +0800, menglong8.dong@xxxxxxxxx wrote:
> > From: Menglong Dong <imagedong@xxxxxxxxxxx>
> >
> > For now, the BPF program of type BPF_PROG_TYPE_TRACING can only be used
> > on the kernel functions whose arguments count less than 6. This is not
> > friendly at all, as too many functions have arguments count more than 6.
> >
> > Therefore, let's enhance it by increasing the function arguments count
> > allowed in arch_prepare_bpf_trampoline(), for now, only x86_64.
> >
> > For the case that we don't need to call origin function, which means
> > without BPF_TRAMP_F_CALL_ORIG, we need only copy the function arguments
> > that stored in the frame of the caller to current frame. The arguments
> > of arg6-argN are stored in "$rbp + 0x18", we need copy them to
> > "$rbp - regs_off + (6 * 8)".
> >
> > For the case with BPF_TRAMP_F_CALL_ORIG, we need prepare the arguments
> > in stack before call origin function, which means we need alloc extra
> > "8 * (arg_count - 6)" memory in the top of the stack. Note, there should
> > not be any data be pushed to the stack before call the origin function.
> > Then, we have to store rbx with 'mov' instead of 'push'.
> >
> > It works well for the FENTRY and FEXIT, I'm not sure if there are other
> > complicated cases.
> >
> > Reviewed-by: Jiang Biao <benbjiang@xxxxxxxxxxx>
> > Signed-off-by: Menglong Dong <imagedong@xxxxxxxxxxx>
> > ---
> > v2:
> > - instead EMIT4 with EMIT3_off32 for "lea" to prevent overflow
>
> could you please describe in more details what's the problem with that?
> you also changed that for 'sub rsp, stack_size'
>

Sorry for the confusion. Take 'sub rsp, stack_size' for example,
in the origin logic, which is:

EMIT4(0x48, 0x83, 0xEC, stack_size)

the imm in the instruction is a signed char. So the maximum
of the imm is 127.

However, now the stack_size is more than 127 if
the count of the function arguments is more than 8.

Therefore, I use:

EMIT3_off32(0x48, 0x81, 0xEC, stack_size)

And the imm in this instruction is signed int.

The same reason for "lea" instruction.

Thanks!
Menglong Dong

> thanks
> jirka
>
>
> > - make MAX_BPF_FUNC_ARGS as the maximum argument count
> > ---
> > arch/x86/net/bpf_jit_comp.c | 96 +++++++++++++++++++++++++++++++------
> > 1 file changed, 81 insertions(+), 15 deletions(-)
> >
> > diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> > index 1056bbf55b17..0e247bb7d6f6 100644
> > --- a/arch/x86/net/bpf_jit_comp.c
> > +++ b/arch/x86/net/bpf_jit_comp.c
> > @@ -1868,7 +1868,7 @@ static void save_regs(const struct btf_func_model *m, u8 **prog, int nr_regs,
> > * mov QWORD PTR [rbp-0x10],rdi
> > * mov QWORD PTR [rbp-0x8],rsi
> > */
> > - for (i = 0, j = 0; i < min(nr_regs, 6); i++) {
> > + for (i = 0, j = 0; i < min(nr_regs, MAX_BPF_FUNC_ARGS); i++) {
> > /* The arg_size is at most 16 bytes, enforced by the verifier. */
> > arg_size = m->arg_size[j];
> > if (arg_size > 8) {
> > @@ -1876,10 +1876,22 @@ static void save_regs(const struct btf_func_model *m, u8 **prog, int nr_regs,
> > next_same_struct = !next_same_struct;
> > }
> >
> > - 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));
> > + if (i <= 5) {
> > + /* store function arguments in regs */
> > + 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));
> > + } else {
> > + /* store function arguments in stack */
> > + emit_ldx(prog, bytes_to_bpf_size(arg_size),
> > + 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,
> > + -(stack_size - i * 8));
> > + }
> >
> > j = next_same_struct ? j : j + 1;
> > }
> > @@ -1913,6 +1925,41 @@ static void restore_regs(const struct btf_func_model *m, u8 **prog, int nr_regs,
> > }
> > }
> >
> > +static void prepare_origin_stack(const struct btf_func_model *m, u8 **prog,
> > + int nr_regs, int stack_size)
> > +{
> > + int i, j, arg_size;
> > + bool next_same_struct = false;
> > +
> > + if (nr_regs <= 6)
> > + return;
> > +
> > + /* Prepare the function arguments in stack before call origin
> > + * function. These arguments must be stored in the top of the
> > + * stack.
> > + */
> > + for (i = 0, j = 0; i < min(nr_regs, MAX_BPF_FUNC_ARGS); 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;
> > + }
> > +
> > + if (i > 5) {
> > + emit_ldx(prog, bytes_to_bpf_size(arg_size),
> > + 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,
> > + -(stack_size - (i - 6) * 8));
> > + }
> > +
> > + j = next_same_struct ? j : j + 1;
> > + }
> > +}
> > +
> > static int invoke_bpf_prog(const struct btf_func_model *m, u8 **pprog,
> > struct bpf_tramp_link *l, int stack_size,
> > int run_ctx_off, bool save_ret)
> > @@ -1938,7 +1985,7 @@ static int invoke_bpf_prog(const struct btf_func_model *m, u8 **pprog,
> > /* arg1: mov rdi, progs[i] */
> > emit_mov_imm64(&prog, BPF_REG_1, (long) p >> 32, (u32) (long) p);
> > /* arg2: lea rsi, [rbp - ctx_cookie_off] */
> > - EMIT4(0x48, 0x8D, 0x75, -run_ctx_off);
> > + EMIT3_off32(0x48, 0x8D, 0xB5, -run_ctx_off);
> >
> > if (emit_rsb_call(&prog, bpf_trampoline_enter(p), prog))
> > return -EINVAL;
> > @@ -1954,7 +2001,7 @@ static int invoke_bpf_prog(const struct btf_func_model *m, u8 **pprog,
> > emit_nops(&prog, 2);
> >
> > /* arg1: lea rdi, [rbp - stack_size] */
> > - EMIT4(0x48, 0x8D, 0x7D, -stack_size);
> > + EMIT3_off32(0x48, 0x8D, 0xBD, -stack_size);
> > /* arg2: progs[i]->insnsi for interpreter */
> > if (!p->jited)
> > emit_mov_imm64(&prog, BPF_REG_2,
> > @@ -1984,7 +2031,7 @@ static int invoke_bpf_prog(const struct btf_func_model *m, u8 **pprog,
> > /* arg2: mov rsi, rbx <- start time in nsec */
> > emit_mov_reg(&prog, true, BPF_REG_2, BPF_REG_6);
> > /* arg3: lea rdx, [rbp - run_ctx_off] */
> > - EMIT4(0x48, 0x8D, 0x55, -run_ctx_off);
> > + EMIT3_off32(0x48, 0x8D, 0x95, -run_ctx_off);
> > if (emit_rsb_call(&prog, bpf_trampoline_exit(p), prog))
> > return -EINVAL;
> >
> > @@ -2136,7 +2183,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
> > void *func_addr)
> > {
> > int i, ret, nr_regs = m->nr_args, stack_size = 0;
> > - int regs_off, nregs_off, ip_off, run_ctx_off;
> > + int regs_off, nregs_off, ip_off, run_ctx_off, arg_stack_off, rbx_off;
> > struct bpf_tramp_links *fentry = &tlinks[BPF_TRAMP_FENTRY];
> > struct bpf_tramp_links *fexit = &tlinks[BPF_TRAMP_FEXIT];
> > struct bpf_tramp_links *fmod_ret = &tlinks[BPF_TRAMP_MODIFY_RETURN];
> > @@ -2150,8 +2197,10 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
> > if (m->arg_flags[i] & BTF_FMODEL_STRUCT_ARG)
> > nr_regs += (m->arg_size[i] + 7) / 8 - 1;
> >
> > - /* x86-64 supports up to 6 arguments. 7+ can be added in the future */
> > - if (nr_regs > 6)
> > + /* x86-64 supports up to MAX_BPF_FUNC_ARGS arguments. 1-6
> > + * are passed through regs, the remains are through stack.
> > + */
> > + if (nr_regs > MAX_BPF_FUNC_ARGS)
> > return -ENOTSUPP;
> >
> > /* Generated trampoline stack layout:
> > @@ -2170,7 +2219,14 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
> > *
> > * RBP - ip_off [ traced function ] BPF_TRAMP_F_IP_ARG flag
> > *
> > + * RBP - rbx_off [ rbx value ] always
> > + *
> > * RBP - run_ctx_off [ bpf_tramp_run_ctx ]
> > + *
> > + * [ stack_argN ] BPF_TRAMP_F_CALL_ORIG
> > + * [ ... ]
> > + * [ stack_arg2 ]
> > + * RBP - arg_stack_off [ stack_arg1 ]
> > */
> >
> > /* room for return value of orig_call or fentry prog */
> > @@ -2190,9 +2246,17 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
> >
> > ip_off = stack_size;
> >
> > + stack_size += 8;
> > + rbx_off = stack_size;
> > +
> > stack_size += (sizeof(struct bpf_tramp_run_ctx) + 7) & ~0x7;
> > run_ctx_off = stack_size;
> >
> > + if (nr_regs > 6 && (flags & BPF_TRAMP_F_CALL_ORIG))
> > + stack_size += (nr_regs - 6) * 8;
> > +
> > + arg_stack_off = stack_size;
> > +
> > if (flags & BPF_TRAMP_F_SKIP_FRAME) {
> > /* skip patched call instruction and point orig_call to actual
> > * body of the kernel function.
> > @@ -2212,8 +2276,9 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
> > x86_call_depth_emit_accounting(&prog, NULL);
> > EMIT1(0x55); /* push rbp */
> > EMIT3(0x48, 0x89, 0xE5); /* mov rbp, rsp */
> > - EMIT4(0x48, 0x83, 0xEC, stack_size); /* sub rsp, stack_size */
> > - EMIT1(0x53); /* push rbx */
> > + EMIT3_off32(0x48, 0x81, 0xEC, stack_size); /* sub rsp, stack_size */
> > + /* mov QWORD PTR [rbp - rbx_off], rbx */
> > + emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_6, -rbx_off);
> >
> > /* Store number of argument registers of the traced function:
> > * mov rax, nr_regs
> > @@ -2262,6 +2327,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
> >
> > if (flags & BPF_TRAMP_F_CALL_ORIG) {
> > restore_regs(m, &prog, nr_regs, regs_off);
> > + prepare_origin_stack(m, &prog, nr_regs, arg_stack_off);
> >
> > if (flags & BPF_TRAMP_F_ORIG_STACK) {
> > emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, 8);
> > @@ -2321,14 +2387,14 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
> > if (save_ret)
> > emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, -8);
> >
> > - EMIT1(0x5B); /* pop rbx */
> > + emit_ldx(&prog, BPF_DW, BPF_REG_6, BPF_REG_FP, -rbx_off);
> > EMIT1(0xC9); /* leave */
> > if (flags & BPF_TRAMP_F_SKIP_FRAME)
> > /* skip our return address and return to parent */
> > EMIT4(0x48, 0x83, 0xC4, 8); /* add rsp, 8 */
> > emit_return(&prog, prog);
> > /* Make sure the trampoline generation logic doesn't overflow */
> > - if (WARN_ON_ONCE(prog > (u8 *)image_end - BPF_INSN_SAFETY)) {
> > + if (prog > (u8 *)image_end - BPF_INSN_SAFETY) {
> > ret = -EFAULT;
> > goto cleanup;
> > }
> > --
> > 2.40.1
> >

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