[PATCH bpf-next v2 04/10] bpf,x86: add ret_off to invoke_bpf()

Menglong Dong
2025-10-22T08:02:32Z

For now, the offset of the return value in trampoline is fixed 8-bytes.
In this commit, we introduce the variable "ret_off" to represent the
offset of the return value. For now, the "ret_off" is just 8. And in the
following patch, we will make it something else to use the room after it.

Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
---
arch/x86/net/bpf_jit_comp.c | 41 +++++++++++++++++++++----------------
1 file changed, 23 insertions(+), 18 deletions(-)

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 389c3a96e2b8..7a604ee9713f 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -2940,7 +2940,7 @@ static void restore_regs(const struct btf_func_model *m, u8 **prog,

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,
+ int run_ctx_off, bool save_ret, int ret_off,
void *image, void *rw_image)
{
u8 *prog = *pprog;
@@ -3005,7 +3005,7 @@ static int invoke_bpf_prog(const struct btf_func_model *m, u8 **pprog,
* value of BPF_PROG_TYPE_STRUCT_OPS prog.
*/
if (save_ret)
- emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -8);
+ emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -ret_off);

/* replace 2 nops with JE insn, since jmp target is known */
jmp_insn[0] = X86_JE;
@@ -3055,7 +3055,7 @@ static int emit_cond_near_jump(u8 **pprog, void *func, void *ip, u8 jmp_cond)

static int invoke_bpf(const struct btf_func_model *m, u8 **pprog,
struct bpf_tramp_links *tl, int stack_size,
- int run_ctx_off, bool save_ret,
+ int run_ctx_off, bool save_ret, int ret_off,
void *image, void *rw_image)
{
int i;
@@ -3063,7 +3063,8 @@ static int invoke_bpf(const struct btf_func_model *m, u8 **pprog,

for (i = 0; i < tl->nr_links; i++) {
if (invoke_bpf_prog(m, &prog, tl->links[i], stack_size,
- run_ctx_off, save_ret, image, rw_image))
+ run_ctx_off, save_ret, ret_off, image,
+ rw_image))
return -EINVAL;
}
*pprog = prog;
@@ -3072,7 +3073,7 @@ static int invoke_bpf(const struct btf_func_model *m, u8 **pprog,

static int invoke_bpf_mod_ret(const struct btf_func_model *m, u8 **pprog,
struct bpf_tramp_links *tl, int stack_size,
- int run_ctx_off, u8 **branches,
+ int run_ctx_off, int ret_off, u8 **branches,
void *image, void *rw_image)
{
u8 *prog = *pprog;
@@ -3082,18 +3083,18 @@ static int invoke_bpf_mod_ret(const struct btf_func_model *m, u8 **pprog,
* Set this to 0 to avoid confusing the program.
*/
emit_mov_imm32(&prog, false, BPF_REG_0, 0);
- emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -8);
+ emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -ret_off);
for (i = 0; i < tl->nr_links; i++) {
if (invoke_bpf_prog(m, &prog, tl->links[i], stack_size, run_ctx_off, true,
- image, rw_image))
+ ret_off, image, rw_image))
return -EINVAL;

- /* mod_ret prog stored return value into [rbp - 8]. Emit:
- * if (*(u64 *)(rbp - 8) != 0)
+ /* mod_ret prog stored return value into [rbp - ret_off]. Emit:
+ * if (*(u64 *)(rbp - ret_off) != 0)
* goto do_fexit;
*/
- /* cmp QWORD PTR [rbp - 0x8], 0x0 */
- EMIT4(0x48, 0x83, 0x7d, 0xf8); EMIT1(0x00);
+ /* cmp QWORD PTR [rbp - ret_off], 0x0 */
+ EMIT4(0x48, 0x83, 0x7d, -ret_off); EMIT1(0x00);

/* Save the location of the branch and Generate 6 nops
* (4 bytes for an offset and 2 bytes for the jump) These nops
@@ -3179,7 +3180,8 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *rw_im
void *func_addr)
{
int i, ret, nr_regs = m->nr_args, stack_size = 0;
- int regs_off, nregs_off, ip_off, run_ctx_off, arg_stack_off, rbx_off;
+ int ret_off, 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];
@@ -3213,7 +3215,7 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *rw_im
* RBP + 8 [ return address ]
* RBP + 0 [ RBP ]
*
- * RBP - 8 [ return value ] BPF_TRAMP_F_CALL_ORIG or
+ * RBP - ret_off [ return value ] BPF_TRAMP_F_CALL_ORIG or
* BPF_TRAMP_F_RET_FENTRY_RET flags
*
* [ reg_argN ] always
@@ -3239,6 +3241,7 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *rw_im
save_ret = flags & (BPF_TRAMP_F_CALL_ORIG | BPF_TRAMP_F_RET_FENTRY_RET);
if (save_ret)
stack_size += 8;
+ ret_off = stack_size;

stack_size += nr_regs * 8;
regs_off = stack_size;
@@ -3341,7 +3344,8 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *rw_im

if (fentry->nr_links) {
if (invoke_bpf(m, &prog, fentry, regs_off, run_ctx_off,
- flags & BPF_TRAMP_F_RET_FENTRY_RET, image, rw_image))
+ flags & BPF_TRAMP_F_RET_FENTRY_RET, ret_off,
+ image, rw_image))
return -EINVAL;
}

@@ -3352,7 +3356,8 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *rw_im
return -ENOMEM;

if (invoke_bpf_mod_ret(m, &prog, fmod_ret, regs_off,
- run_ctx_off, branches, image, rw_image)) {
+ run_ctx_off, ret_off, branches,
+ image, rw_image)) {
ret = -EINVAL;
goto cleanup;
}
@@ -3380,7 +3385,7 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *rw_im
}
}
/* remember return value in a stack for bpf prog to access */
- emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -8);
+ emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -ret_off);
im->ip_after_call = image + (prog - (u8 *)rw_image);
emit_nops(&prog, X86_PATCH_SIZE);
}
@@ -3403,7 +3408,7 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *rw_im

if (fexit->nr_links) {
if (invoke_bpf(m, &prog, fexit, regs_off, run_ctx_off,
- false, image, rw_image)) {
+ false, ret_off, image, rw_image)) {
ret = -EINVAL;
goto cleanup;
}
@@ -3433,7 +3438,7 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *rw_im

/* restore return value of orig_call or fentry prog back into RAX */
if (save_ret)
- emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, -8);
+ emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, -ret_off);

emit_ldx(&prog, BPF_DW, BPF_REG_6, BPF_REG_FP, -rbx_off);
EMIT1(0xC9); /* leave */
--
2.51.1.dirty