From: Yonghong Song yhs@fb.com
[ Upstream commit 63d78b7e8ca2d0eb8c687a355fa19d01b6fcc723 ]
With latest llvm17, selftest fexit_bpf2bpf/func_replace_return_code has the following verification failure:
0: R1=ctx(off=0,imm=0) R10=fp0 ; int connect_v4_prog(struct bpf_sock_addr *ctx) 0: (bf) r7 = r1 ; R1=ctx(off=0,imm=0) R7_w=ctx(off=0,imm=0) 1: (b4) w6 = 0 ; R6_w=0 ; memset(&tuple.ipv4.saddr, 0, sizeof(tuple.ipv4.saddr)); ... ; return do_bind(ctx) ? 1 : 0; 179: (bf) r1 = r7 ; R1=ctx(off=0,imm=0) R7=ctx(off=0,imm=0) 180: (85) call pc+147 Func#3 is global and valid. Skipping. 181: R0_w=scalar() 181: (bc) w6 = w0 ; R0_w=scalar() R6_w=scalar(umax=4294967295,var_off=(0x0; 0xffffffff)) 182: (05) goto pc-129 ; } 54: (bc) w0 = w6 ; R0_w=scalar(umax=4294967295,var_off=(0x0; 0xffffffff)) R6_w=scalar(umax=4294967295,var_off=(0x0; 0xffffffff)) 55: (95) exit At program exit the register R0 has value (0x0; 0xffffffff) should have been in (0x0; 0x1) processed 281 insns (limit 1000000) max_states_per_insn 1 total_states 26 peak_states 26 mark_read 13 -- END PROG LOAD LOG -- libbpf: prog 'connect_v4_prog': failed to load: -22
The corresponding source code:
__attribute__ ((noinline)) int do_bind(struct bpf_sock_addr *ctx) { struct sockaddr_in sa = {};
sa.sin_family = AF_INET; sa.sin_port = bpf_htons(0); sa.sin_addr.s_addr = bpf_htonl(SRC_REWRITE_IP4);
if (bpf_bind(ctx, (struct sockaddr *)&sa, sizeof(sa)) != 0) return 0;
return 1; } ... SEC("cgroup/connect4") int connect_v4_prog(struct bpf_sock_addr *ctx) { ... return do_bind(ctx) ? 1 : 0; }
Insn 180 is a call to 'do_bind'. The call's return value is also the return value for the program. Since do_bind() returns 0/1, so it is legitimate for compiler to optimize 'return do_bind(ctx) ? 1 : 0' to 'return do_bind(ctx)'. However, such optimization breaks verifier as the return value of 'do_bind()' is marked as any scalar which violates the requirement of prog return value 0/1.
There are two ways to fix this problem, (1) changing 'return 1' in do_bind() to e.g. 'return 10' so the compiler has to do 'do_bind(ctx) ? 1 :0', or (2) suggested by Andrii, marking do_bind() with __weak attribute so the compiler cannot make any assumption on do_bind() return value.
This patch adopted adding __weak approach which is simpler and more resistant to potential compiler optimizations.
Suggested-by: Andrii Nakryiko andrii@kernel.org Signed-off-by: Yonghong Song yhs@fb.com Signed-off-by: Andrii Nakryiko andrii@kernel.org Link: https://lore.kernel.org/bpf/20230310012410.2920570-1-yhs@fb.com --- tools/testing/selftests/bpf/progs/connect4_prog.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/tools/testing/selftests/bpf/progs/connect4_prog.c b/tools/testing/selftests/bpf/progs/connect4_prog.c index ec25371de789..7ef49ec04838 100644 --- a/tools/testing/selftests/bpf/progs/connect4_prog.c +++ b/tools/testing/selftests/bpf/progs/connect4_prog.c @@ -32,7 +32,7 @@ #define IFNAMSIZ 16 #endif
-__attribute__ ((noinline)) +__attribute__ ((noinline)) __weak int do_bind(struct bpf_sock_addr *ctx) { struct sockaddr_in sa = {};