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bpf: Emit timed may_goto for JITed progs only

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The timed form of may_goto calls arch_bpf_timed_may_goto(). This function
takes its argument and returns its result in BPF_REG_AX, in a register of
the cpu. Only JITed code can call it.

The verifier emits the timed form when the arch has a JIT that supports it.
It does not look at the prog. With CONFIG_BPF_JIT_ALWAYS_ON=n the
interpreter can run such a prog: the JIT is off, or it fails. The
interpreter calls the function like a helper. The function takes what the C
code left in that register for a pointer, and bpf_check_timed_may_goto()
reads and writes through it.

Emit the timed form only when the prog asks for a JIT, and make the JIT
mandatory for such a prog. All other progs get the form that counts down
from BPF_MAX_LOOPS.

One test of verifier_may_goto_1 shows the insns of the timed form. Load it
only when progs are JITed.

Fixes: e723608bf4 ("bpf: Add verifier support for timed may_goto")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261031215543.682284-2-alexei.starovoitov@gmail.com
This commit is contained in:
Alexei Starovoitov 2026-10-31 21:55:35 +00:00
parent 29f85bba33
commit 1ddffb57b3
No known key found for this signature in database
2 changed files with 7 additions and 3 deletions
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9
kernel/bpf/fixups.c
View File

@ -1823,6 +1823,8 @@ int bpf_do_misc_fixups(struct bpf_verifier_env *env)
struct bpf_subprog_info *subprogs = env->subprog_info;
u16 stack_depth = subprogs[cur_subprog].stack_depth;
u16 stack_depth_extra = 0;
/* arch_bpf_timed_may_goto() takes and returns AX in a cpu register: for JITs only */
bool timed_may_goto = prog->jit_requested && bpf_jit_supports_timed_may_goto();
if (env->seen_exception && !env->exception_callback_subprog) {
struct bpf_insn *patch = insn_buf;
@ -2078,7 +2080,7 @@ int bpf_do_misc_fixups(struct bpf_verifier_env *env)
goto next_insn;
}
if (bpf_is_may_goto_insn(insn) && bpf_jit_supports_timed_may_goto()) {
if (bpf_is_may_goto_insn(insn) && timed_may_goto) {
int stack_off_cnt = -stack_depth - 16;
/*
* AX is used as an argument to pass in stack_off_cnt
@ -2108,6 +2110,7 @@ int bpf_do_misc_fixups(struct bpf_verifier_env *env)
* after subtraction, rinse and repeat.
*/
stack_depth_extra = 16;
prog->jit_required = 1;
cnt = may_goto_expand(insn_buf, insn->off, stack_off_cnt,
tail, ARRAY_SIZE(tail));
@ -2730,7 +2733,7 @@ int bpf_do_misc_fixups(struct bpf_verifier_env *env)
env->prog->aux->stack_depth = subprogs[0].stack_depth;
for (i = 0; i < env->subprog_cnt; i++) {
int delta = bpf_jit_supports_timed_may_goto() ? 2 : 1;
int delta = timed_may_goto ? 2 : 1;
int subprog_start = subprogs[i].start;
int stack_slots = subprogs[i].stack_extra / 8;
int slots = delta, cnt = 0;
@ -2744,7 +2747,7 @@ int bpf_do_misc_fixups(struct bpf_verifier_env *env)
}
stack_depth = subprogs[i].stack_depth;
if (bpf_jit_supports_timed_may_goto()) {
if (timed_may_goto) {
insn_buf[cnt++] = BPF_ST_MEM(BPF_DW, BPF_REG_FP, -stack_depth,
BPF_MAX_TIMED_LOOPS);
insn_buf[cnt++] = BPF_ST_MEM(BPF_DW, BPF_REG_FP, -stack_depth + 8, 0);

1
tools/testing/selftests/bpf/progs/verifier_may_goto_1.c
View File

@ -82,6 +82,7 @@ __naked void may_goto_batch_1(void)
SEC("raw_tp")
__description("may_goto batch with offsets 2/0")
__load_if_JITed()
__arch_x86_64
__arch_s390x
__arch_arm64