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authorAlexei Starovoitov <ast@kernel.org>2024-03-06 06:19:26 +0300
committerAndrii Nakryiko <andrii@kernel.org>2024-03-07 02:17:31 +0300
commit011832b97b311bb9e3c27945bc0d1089a14209c9 (patch)
tree895a1b974a1b6edcd101286dcf3436fffad046b2 /tools/include
parent9a9d1d36050e486822dc54990c896761b04e7446 (diff)
downloadlinux-011832b97b311bb9e3c27945bc0d1089a14209c9.tar.xz
bpf: Introduce may_goto instruction
Introduce may_goto instruction that from the verifier pov is similar to open coded iterators bpf_for()/bpf_repeat() and bpf_loop() helper, but it doesn't iterate any objects. In assembly 'may_goto' is a nop most of the time until bpf runtime has to terminate the program for whatever reason. In the current implementation may_goto has a hidden counter, but other mechanisms can be used. For programs written in C the later patch introduces 'cond_break' macro that combines 'may_goto' with 'break' statement and has similar semantics: cond_break is a nop until bpf runtime has to break out of this loop. It can be used in any normal "for" or "while" loop, like for (i = zero; i < cnt; cond_break, i++) { The verifier recognizes that may_goto is used in the program, reserves additional 8 bytes of stack, initializes them in subprog prologue, and replaces may_goto instruction with: aux_reg = *(u64 *)(fp - 40) if aux_reg == 0 goto pc+off aux_reg -= 1 *(u64 *)(fp - 40) = aux_reg may_goto instruction can be used by LLVM to implement __builtin_memcpy, __builtin_strcmp. may_goto is not a full substitute for bpf_for() macro. bpf_for() doesn't have induction variable that verifiers sees, so 'i' in bpf_for(i, 0, 100) is seen as imprecise and bounded. But when the code is written as: for (i = 0; i < 100; cond_break, i++) the verifier see 'i' as precise constant zero, hence cond_break (aka may_goto) doesn't help to converge the loop. A static or global variable can be used as a workaround: static int zero = 0; for (i = zero; i < 100; cond_break, i++) // works! may_goto works well with arena pointers that don't need to be bounds checked on access. Load/store from arena returns imprecise unbounded scalar and loops with may_goto pass the verifier. Reserve new opcode BPF_JMP | BPF_JCOND for may_goto insn. JCOND stands for conditional pseudo jump. Since goto_or_nop insn was proposed, it may use the same opcode. may_goto vs goto_or_nop can be distinguished by src_reg: code = BPF_JMP | BPF_JCOND src_reg = 0 - may_goto src_reg = 1 - goto_or_nop Signed-off-by: Alexei Starovoitov <ast@kernel.org> Signed-off-by: Andrii Nakryiko <andrii@kernel.org> Acked-by: Andrii Nakryiko <andrii@kernel.org> Acked-by: Eduard Zingerman <eddyz87@gmail.com> Acked-by: John Fastabend <john.fastabend@gmail.com> Tested-by: John Fastabend <john.fastabend@gmail.com> Link: https://lore.kernel.org/bpf/20240306031929.42666-2-alexei.starovoitov@gmail.com
Diffstat (limited to 'tools/include')
-rw-r--r--tools/include/uapi/linux/bpf.h5
1 files changed, 5 insertions, 0 deletions
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index a241f407c234..85ec7fc799d7 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -42,6 +42,7 @@
#define BPF_JSGE 0x70 /* SGE is signed '>=', GE in x86 */
#define BPF_JSLT 0xc0 /* SLT is signed, '<' */
#define BPF_JSLE 0xd0 /* SLE is signed, '<=' */
+#define BPF_JCOND 0xe0 /* conditional pseudo jumps: may_goto, goto_or_nop */
#define BPF_CALL 0x80 /* function call */
#define BPF_EXIT 0x90 /* function return */
@@ -50,6 +51,10 @@
#define BPF_XCHG (0xe0 | BPF_FETCH) /* atomic exchange */
#define BPF_CMPXCHG (0xf0 | BPF_FETCH) /* atomic compare-and-write */
+enum bpf_cond_pseudo_jmp {
+ BPF_MAY_GOTO = 0,
+};
+
/* Register numbers */
enum {
BPF_REG_0 = 0,