[PATCH v2] libbpf: add GCC support for bpf_tail_call_static

James Hilliard
2022-08-29T21:05:57Z

The bpf_tail_call_static function is currently not defined unless
using clang >= 8.

To support bpf_tail_call_static on GCC we can check if __clang__ is
not defined to enable bpf_tail_call_static.

We need to use GCC assembly syntax when the compiler does not define
__clang__ as LLVM inline assembly is not fully compatible with GCC.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
---
Changes v1 -> v2:
- drop __BPF__ check as GCC now defines __bpf__
---
tools/lib/bpf/bpf_helpers.h | 19 +++++++++++++------
1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
index 7349b16b8e2f..867b734839dd 100644
--- a/tools/lib/bpf/bpf_helpers.h
+++ b/tools/lib/bpf/bpf_helpers.h
@@ -131,7 +131,7 @@
/*
* Helper function to perform a tail call with a constant/immediate map slot.
*/
-#if __clang_major__ >= 8 && defined(__bpf__)
+#if (!defined(__clang__) || __clang_major__ >= 8) && defined(__bpf__)
static __always_inline void
bpf_tail_call_static(void *ctx, const void *map, const __u32 slot)
{
@@ -139,8 +139,8 @@ bpf_tail_call_static(void *ctx, const void *map, const __u32 slot)
__bpf_unreachable();

/*
- * Provide a hard guarantee that LLVM won't optimize setting r2 (map
- * pointer) and r3 (constant map index) from _different paths_ ending
+ * Provide a hard guarantee that the compiler won't optimize setting r2
+ * (map pointer) and r3 (constant map index) from _different paths_ ending
* up at the _same_ call insn as otherwise we won't be able to use the
* jmpq/nopl retpoline-free patching by the x86-64 JIT in the kernel
* given they mismatch. See also d2e4c1e6c294 ("bpf: Constant map key
@@ -148,12 +148,19 @@ bpf_tail_call_static(void *ctx, const void *map, const __u32 slot)
*
* Note on clobber list: we need to stay in-line with BPF calling
* convention, so even if we don't end up using r0, r4, r5, we need
- * to mark them as clobber so that LLVM doesn't end up using them
- * before / after the call.
+ * to mark them as clobber so that the compiler doesn't end up using
+ * them before / after the call.
*/
- asm volatile("r1 = %[ctx]\n\t"
+ asm volatile(
+#ifdef __clang__
+ "r1 = %[ctx]\n\t"
"r2 = %[map]\n\t"
"r3 = %[slot]\n\t"
+#else
+ "mov %%r1,%[ctx]\n\t"
+ "mov %%r2,%[map]\n\t"
+ "mov %%r3,%[slot]\n\t"
+#endif
"call 12"
:: [ctx]"r"(ctx), [map]"r"(map), [slot]"i"(slot)
: "r0", "r1", "r2", "r3", "r4", "r5");
--
2.34.1