From 3193c0836f203a91bef96d88c64cccf0be090d9c Mon Sep 17 00:00:00 2001 From: Josh Poimboeuf Date: Wed, 17 Jul 2019 20:36:45 -0500 Subject: bpf: Disable GCC -fgcse optimization for ___bpf_prog_run() On x86-64, with CONFIG_RETPOLINE=n, GCC's "global common subexpression elimination" optimization results in ___bpf_prog_run()'s jumptable code changing from this: select_insn: jmp *jumptable(, %rax, 8) ... ALU64_ADD_X: ... jmp *jumptable(, %rax, 8) ALU_ADD_X: ... jmp *jumptable(, %rax, 8) to this: select_insn: mov jumptable, %r12 jmp *(%r12, %rax, 8) ... ALU64_ADD_X: ... jmp *(%r12, %rax, 8) ALU_ADD_X: ... jmp *(%r12, %rax, 8) The jumptable address is placed in a register once, at the beginning of the function. The function execution can then go through multiple indirect jumps which rely on that same register value. This has a few issues: 1) Objtool isn't smart enough to be able to track such a register value across multiple recursive indirect jumps through the jump table. 2) With CONFIG_RETPOLINE enabled, this optimization actually results in a small slowdown. I measured a ~4.7% slowdown in the test_bpf "tcpdump port 22" selftest. This slowdown is actually predicted by the GCC manual: Note: When compiling a program using computed gotos, a GCC extension, you may get better run-time performance if you disable the global common subexpression elimination pass by adding -fno-gcse to the command line. So just disable the optimization for this function. Fixes: e55a73251da3 ("bpf: Fix ORC unwinding in non-JIT BPF code") Reported-by: Randy Dunlap Signed-off-by: Josh Poimboeuf Signed-off-by: Thomas Gleixner Acked-by: Alexei Starovoitov Acked-by: Peter Zijlstra (Intel) Link: https://lkml.kernel.org/r/30c3ca29ba037afcbd860a8672eef0021addf9fe.1563413318.git.jpoimboe@redhat.com --- kernel/bpf/core.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'kernel') diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c index 7e98f36a14e2..8191a7db2777 100644 --- a/kernel/bpf/core.c +++ b/kernel/bpf/core.c @@ -1295,7 +1295,7 @@ bool bpf_opcode_in_insntable(u8 code) * * Decode and execute eBPF instructions. */ -static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack) +static u64 __no_fgcse ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack) { #define BPF_INSN_2_LBL(x, y) [BPF_##x | BPF_##y] = &&x##_##y #define BPF_INSN_3_LBL(x, y, z) [BPF_##x | BPF_##y | BPF_##z] = &&x##_##y##_##z -- cgit v1.2.3