[BPF] Add support for may_goto insn (#85358)
Alexei added may_goto insn in [1]. The asm syntax for may_goto looks
like
may_goto <label>
The instruction represents a conditional branch but the condition is
implicit. Later in bpf kernel verifier, the 'may_goto <label>' insn will
be rewritten with an explicit condition. The encoding of 'may_goto' insn
is enforced in [2] and is also implemented in this patch.
In [3], 'may_goto' insn is encoded with raw bytes. I made the following
change
```
--- a/tools/testing/selftests/bpf/bpf_experimental.h
+++ b/tools/testing/selftests/bpf/bpf_experimental.h
@@ -328,10 +328,7 @@ l_true: \
#define cond_break \
({ __label__ l_break, l_continue; \
- asm volatile goto("1:.byte 0xe5; \
- .byte 0; \
- .long ((%l[l_break] - 1b - 8) / 8) & 0xffff; \
- .short 0" \
+ asm volatile goto("may_goto %l[l_break]" \
:::: l_break); \
goto l_continue; \
l_break: break;
```
and ran the selftest with the latest llvm with this patch. All tests are
passed.
[1]
https://lore.kernel.org/bpf/20240306031929.42666-1-alexei.starovoitov@gmail.com/
[2]
https://lore.kernel.org/bpf/20240306031929.42666-2-alexei.starovoitov@gmail.com/
[3]
https://lore.kernel.org/bpf/20240306031929.42666-4-alexei.starovoitov@gmail.com/
