bpf: Don't rely on GCC attribute((optimize)) to disable GCSE

Commit 3193c0836 ("bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()") introduced a __no_fgcse macro that expands to a function scope __attribute__((optimize("-fno-gcse"))), to disable a GCC specific optimization that was causing trouble on x86 builds, and was not expected to have any positive effect in the first place. However, as the GCC manual documents, __attribute__((optimize)) is not for production use, and results in all other optimization options to be forgotten for the function in question. This can cause all kinds of trouble, but in one particular reported case, it causes -fno-asynchronous-unwind-tables to be disregarded, resulting in .eh_frame info to be emitted for the function. This reverts commit 3193c0836, and instead, it disables the -fgcse optimization for the entire source file, but only when building for X86 using GCC with CONFIG_BPF_JIT_ALWAYS_ON disabled. Note that the original commit states that CONFIG_RETPOLINE=n triggers the issue, whereas CONFIG_RETPOLINE=y performs better without the optimization, so it is kept disabled in both cases.

Ard Biesheuvel committed 6 years ago

Commit 080b6f41 parent 0698ac64 files · +6 −8Browse files

Unified · Split

4 files

include/linux/compiler-gcc.h +0 −2
include/linux/compiler_types.h +0 −4
kernel/bpf/Makefile +5 −1
kernel/bpf/core.c +1 −1

include/linux/compiler-gcc.h+0−2

@@ -175,5 +175,3 @@ · 174 unmodified lines
175	175	#else
176	176	#define __diag_GCC_8(s)
177	177	#endif
178		−
179		−#define __no_fgcse __attribute__((optimize("-fno-gcse")))

include/linux/compiler_types.h+0−4

@@ -247,10 +247,6 @@ struct ftrace_likely_data { · 246 unmodified lines
247	247	#define asm_inline asm
248	248	#endif
249	249
250		−#ifndef __no_fgcse
251		−# define __no_fgcse
252		−#endif
253		−
254	250	/* Are two types/vars the same type (ignoring qualifiers)? */
255	251	#define __same_type(a, b) __builtin_types_compatible_p(typeof(a), typeof(b))
256	252

kernel/bpf/Makefile+5−1

@@ -1,6 +1,10 @@
1	1	# SPDX-License-Identifier: GPL-2.0
2	2	obj-y := core.o
3		−CFLAGS_core.o += $(call cc-disable-warning, override-init)
	3	+ifneq ($(CONFIG_BPF_JIT_ALWAYS_ON),y)
	4	+# ___bpf_prog_run() needs GCSE disabled on x86; see 3193c0836f203 for details
	5	+cflags-nogcse-$(CONFIG_X86)$(CONFIG_CC_IS_GCC) := -fno-gcse
	6	+endif
	7	+CFLAGS_core.o += $(call cc-disable-warning, override-init) $(cflags-nogcse-yy)
4	8
5	9	obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o tnum.o bpf_iter.o map_iter.o task_iter.o prog_iter.o
6	10	obj-$(CONFIG_BPF_SYSCALL) += hashtab.o arraymap.o percpu_freelist.o bpf_lru_list.o lpm_trie.o map_in_map.o

kernel/bpf/core.c+1−1

@@ -1369,7 +1369,7 @@ u64 __weak bpf_probe_read_kernel(void dst, u32 size, const void unsafe_ptr) · 1368 unmodified lines
1369	1369	*
1370	1370	* Decode and execute eBPF instructions.
1371	1371	*/
1372		−static u64 __no_fgcse ___bpf_prog_run(u64 regs, const struct bpf_insn insn, u64 *stack)
	1372	+static u64 ___bpf_prog_run(u64 regs, const struct bpf_insn insn, u64 *stack)
1373	1373	{
1374	1374	#define BPF_INSN_2_LBL(x, y) [BPF_##x \| BPF_##y] = &&x##_##y
1375	1375	#define BPF_INSN_3_LBL(x, y, z) [BPF_##x \| BPF_##y \| BPF_##z] = &&x##_##y##_##z

bpf: Don't rely on GCC attribute((optimize)) to disable GCSE

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