[PATCH bpf-next v2] bpf, x86: Simplify the parsing logic of structure parameters

From: Pu Lehui
Date: Wed Jan 04 2023 - 23:12:49 EST

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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>
Acked-by: Yonghong Song <yhs@xxxxxx>
---

v2:
- renaming flip flag and add comment for better understanding.
- add ACK by Yonghong.

v1:
https://lore.kernel.org/bpf/20230103013158.1945869-1-pulehui@xxxxxxxxxxxxxxx

arch/x86/net/bpf_jit_comp.c | 101 +++++++++++++++++-------------------
1 file changed, 48 insertions(+), 53 deletions(-)

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index e3e2b57e4e13..ecdfc7bc45d2 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -1839,62 +1839,59 @@ 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 next_same_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++) {
+ /* 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];
+ next_same_struct = !next_same_struct;
}

- 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 = next_same_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 next_same_struct = false;

/* 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++) {
+ /* 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];
+ next_same_struct = !next_same_struct;
}

- 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 = next_same_struct ? j : j + 1;
}
}

@@ -2120,8 +2117,8 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
struct bpf_tramp_links *tlinks,
void *func_addr)
{
- int ret, i, nr_args = m->nr_args, extra_nregs = 0;
- int regs_off, ip_off, args_off, stack_size = nr_args * 8, run_ctx_off;
+ int i, ret, nr_regs = m->nr_args, stack_size = 0;
+ int regs_off, nregs_off, ip_off, run_ctx_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];
@@ -2130,17 +2127,14 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
u8 *prog;
bool save_ret;

- /* x86-64 supports up to 6 arguments. 7+ can be added in the future */
- if (nr_args > 6)
- return -ENOTSUPP;
-
- for (i = 0; i < MAX_BPF_FUNC_ARGS; i++) {
+ /* extra registers for struct arguments */
+ for (i = 0; i < m->nr_args; i++)
if (m->arg_flags[i] & BTF_FMODEL_STRUCT_ARG)
- extra_nregs += (m->arg_size[i] + 7) / 8 - 1;
- }
- if (nr_args + extra_nregs > 6)
+ 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)
return -ENOTSUPP;
- stack_size += extra_nregs * 8;

/* Generated trampoline stack layout:
*
@@ -2154,7 +2148,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
* [ ... ]
* RBP - regs_off [ reg_arg1 ] program's ctx pointer
*
- * RBP - args_off [ arg regs count ] always
+ * RBP - nregs_off [ regs count ] always
*
* RBP - ip_off [ traced function ] BPF_TRAMP_F_IP_ARG flag
*
@@ -2166,11 +2160,12 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
if (save_ret)
stack_size += 8;

+ stack_size += nr_regs * 8;
regs_off = stack_size;

- /* args count */
+ /* regs count */
stack_size += 8;
- args_off = stack_size;
+ nregs_off = stack_size;

if (flags & BPF_TRAMP_F_IP_ARG)
stack_size += 8; /* room for IP address argument */
@@ -2198,11 +2193,11 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
EMIT1(0x53); /* push rbx */

/* Store number of argument registers of the traced function:
- * mov rax, nr_args + extra_nregs
- * mov QWORD PTR [rbp - args_off], rax
+ * mov rax, nr_regs
+ * mov QWORD PTR [rbp - nregs_off], rax
*/
- emit_mov_imm64(&prog, BPF_REG_0, 0, (u32) nr_args + extra_nregs);
- emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -args_off);
+ emit_mov_imm64(&prog, BPF_REG_0, 0, (u32) nr_regs);
+ emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -nregs_off);

if (flags & BPF_TRAMP_F_IP_ARG) {
/* Store IP address of the traced function:
@@ -2213,7 +2208,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -ip_off);
}

- save_regs(m, &prog, nr_args, regs_off);
+ save_regs(m, &prog, nr_regs, regs_off);

if (flags & BPF_TRAMP_F_CALL_ORIG) {
/* arg1: mov rdi, im */
@@ -2243,7 +2238,7 @@ 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_args, regs_off);
+ restore_regs(m, &prog, nr_regs, regs_off);

if (flags & BPF_TRAMP_F_ORIG_STACK) {
emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, 8);
@@ -2284,7 +2279,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
}

if (flags & BPF_TRAMP_F_RESTORE_REGS)
- restore_regs(m, &prog, nr_args, regs_off);
+ restore_regs(m, &prog, nr_regs, regs_off);

/* This needs to be done regardless. If there were fmod_ret programs,
* the return value is only updated on the stack and still needs to be
--
2.25.1

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