Merge branch 'bpf-allow-may_goto-0-instruction'

Yonghong Song says:

====================
Emil Tsalapatis from Meta reported such a case where 'may_goto 0' insn is
generated by clang-19 compiler and this caused verification failure
since 'may_goto 0' is rejected by verifier.

In fact, 'may_goto 0' insn is actually a no-op and it won't hurt
verification. The only side effect is that the verifier will convert
the insn to a sequence of codes like
   /* r10 - 8 stores the implicit loop count */
   r11 = *(u64 *)(r10 -8)
   if r11 == 0x0 goto pc+2
   r11 -= 1
   *(u64 *)(r10 -8) = r11

With this patch set 'may_goto 0' insns are allowed in verification which
also removes those insns.

Changelogs:
  v1 -> v2:
    - Instead of a separate function, removing 'may_goto 0' in existing
      func opt_remove_nops().
====================

Link: https://patch.msgid.link/20250118192019.2123689-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: Alexei Starovoitov

kernel/bpf/verifier.c

@@ -15972,9 +15972,8 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
 
 		if (insn->code != (BPF_JMP | BPF_JCOND) ||
 		    insn->src_reg != BPF_MAY_GOTO ||
-		    insn->dst_reg || insn->imm || insn->off == 0) {
-			verbose(env, "invalid may_goto off %d imm %d\n",
-				insn->off, insn->imm);
+		    insn->dst_reg || insn->imm) {
+			verbose(env, "invalid may_goto imm %d\n", insn->imm);
 			return -EINVAL;
 		}
 		prev_st = find_prev_entry(env, cur_st->parent, idx);
@@ -20185,23 +20184,28 @@ static int opt_remove_dead_code(struct bpf_verifier_env *env)
 }
 
 static const struct bpf_insn NOP = BPF_JMP_IMM(BPF_JA, 0, 0, 0);
+static const struct bpf_insn MAY_GOTO_0 = BPF_RAW_INSN(BPF_JMP | BPF_JCOND, 0, 0, 0, 0);
 
 static int opt_remove_nops(struct bpf_verifier_env *env)
 {
-	const struct bpf_insn ja = NOP;
 	struct bpf_insn *insn = env->prog->insnsi;
 	int insn_cnt = env->prog->len;
+	bool is_may_goto_0, is_ja;
 	int i, err;
 
 	for (i = 0; i < insn_cnt; i++) {
-		if (memcmp(&insn[i], &ja, sizeof(ja)))
+		is_may_goto_0 = !memcmp(&insn[i], &MAY_GOTO_0, sizeof(MAY_GOTO_0));
+		is_ja = !memcmp(&insn[i], &NOP, sizeof(NOP));
+
+		if (!is_may_goto_0 && !is_ja)
 			continue;
 
 		err = verifier_remove_insns(env, i, 1);
 		if (err)
 			return err;
 		insn_cnt--;
-		i--;
+		/* Go back one insn to catch may_goto +1; may_goto +0 sequence */
+		i -= (is_may_goto_0 && i > 0) ? 2 : 1;
 	}
 
 	return 0;

tools/testing/selftests/bpf/prog_tests/verifier.c

@@ -52,6 +52,8 @@
 #include "verifier_map_ptr_mixing.skel.h"
 #include "verifier_map_ret_val.skel.h"
 #include "verifier_masking.skel.h"
+#include "verifier_may_goto_1.skel.h"
+#include "verifier_may_goto_2.skel.h"
 #include "verifier_meta_access.skel.h"
 #include "verifier_movsx.skel.h"
 #include "verifier_mtu.skel.h"
@@ -182,6 +184,8 @@ void test_verifier_map_ptr(void)              { RUN(verifier_map_ptr); }
 void test_verifier_map_ptr_mixing(void)       { RUN(verifier_map_ptr_mixing); }
 void test_verifier_map_ret_val(void)          { RUN(verifier_map_ret_val); }
 void test_verifier_masking(void)              { RUN(verifier_masking); }
+void test_verifier_may_goto_1(void)           { RUN(verifier_may_goto_1); }
+void test_verifier_may_goto_2(void)           { RUN(verifier_may_goto_2); }
 void test_verifier_meta_access(void)          { RUN(verifier_meta_access); }
 void test_verifier_movsx(void)                 { RUN(verifier_movsx); }
 void test_verifier_netfilter_ctx(void)        { RUN(verifier_netfilter_ctx); }

tools/testing/selftests/bpf/progs/verifier_may_goto_1.c

@@ -0,0 +1,97 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2025 Meta Platforms, Inc. and affiliates. */
+
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include "../../../include/linux/filter.h"
+#include "bpf_misc.h"
+
+SEC("raw_tp")
+__description("may_goto 0")
+__arch_x86_64
+__xlated("0: r0 = 1")
+__xlated("1: exit")
+__success
+__naked void may_goto_simple(void)
+{
+	asm volatile (
+	".8byte %[may_goto];"
+	"r0 = 1;"
+	".8byte %[may_goto];"
+	"exit;"
+	:
+	: __imm_insn(may_goto, BPF_RAW_INSN(BPF_JMP | BPF_JCOND, 0, 0, 0 /* offset */, 0))
+	: __clobber_all);
+}
+
+SEC("raw_tp")
+__description("batch 2 of may_goto 0")
+__arch_x86_64
+__xlated("0: r0 = 1")
+__xlated("1: exit")
+__success
+__naked void may_goto_batch_0(void)
+{
+	asm volatile (
+	".8byte %[may_goto1];"
+	".8byte %[may_goto1];"
+	"r0 = 1;"
+	".8byte %[may_goto1];"
+	".8byte %[may_goto1];"
+	"exit;"
+	:
+	: __imm_insn(may_goto1, BPF_RAW_INSN(BPF_JMP | BPF_JCOND, 0, 0, 0 /* offset */, 0))
+	: __clobber_all);
+}
+
+SEC("raw_tp")
+__description("may_goto batch with offsets 2/1/0")
+__arch_x86_64
+__xlated("0: r0 = 1")
+__xlated("1: exit")
+__success
+__naked void may_goto_batch_1(void)
+{
+	asm volatile (
+	".8byte %[may_goto1];"
+	".8byte %[may_goto2];"
+	".8byte %[may_goto3];"
+	"r0 = 1;"
+	".8byte %[may_goto1];"
+	".8byte %[may_goto2];"
+	".8byte %[may_goto3];"
+	"exit;"
+	:
+	: __imm_insn(may_goto1, BPF_RAW_INSN(BPF_JMP | BPF_JCOND, 0, 0, 2 /* offset */, 0)),
+	  __imm_insn(may_goto2, BPF_RAW_INSN(BPF_JMP | BPF_JCOND, 0, 0, 1 /* offset */, 0)),
+	  __imm_insn(may_goto3, BPF_RAW_INSN(BPF_JMP | BPF_JCOND, 0, 0, 0 /* offset */, 0))
+	: __clobber_all);
+}
+
+SEC("raw_tp")
+__description("may_goto batch with offsets 2/0")
+__arch_x86_64
+__xlated("0: *(u64 *)(r10 -8) = 8388608")
+__xlated("1: r11 = *(u64 *)(r10 -8)")
+__xlated("2: if r11 == 0x0 goto pc+3")
+__xlated("3: r11 -= 1")
+__xlated("4: *(u64 *)(r10 -8) = r11")
+__xlated("5: r0 = 1")
+__xlated("6: r0 = 2")
+__xlated("7: exit")
+__success
+__naked void may_goto_batch_2(void)
+{
+	asm volatile (
+	".8byte %[may_goto1];"
+	".8byte %[may_goto3];"
+	"r0 = 1;"
+	"r0 = 2;"
+	"exit;"
+	:
+	: __imm_insn(may_goto1, BPF_RAW_INSN(BPF_JMP | BPF_JCOND, 0, 0, 2 /* offset */, 0)),
+	  __imm_insn(may_goto3, BPF_RAW_INSN(BPF_JMP | BPF_JCOND, 0, 0, 0 /* offset */, 0))
+	: __clobber_all);
+}
+
+char _license[] SEC("license") = "GPL";

tools/testing/selftests/bpf/progs/verifier_may_goto_2.c

@@ -0,0 +1,28 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2025 Meta Platforms, Inc. and affiliates. */
+
+#include "bpf_misc.h"
+#include "bpf_experimental.h"
+
+int gvar;
+
+SEC("raw_tp")
+__description("C code with may_goto 0")
+__success
+int may_goto_c_code(void)
+{
+	int i, tmp[3];
+
+	for (i = 0; i < 3 && can_loop; i++)
+		tmp[i] = 0;
+
+	for (i = 0; i < 3 && can_loop; i++)
+		tmp[i] = gvar - i;
+
+	for (i = 0; i < 3 && can_loop; i++)
+		gvar += tmp[i];
+
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";