[PATCH v2] libbpf: define bpf_tail_call_static when __clang__ is not defined

[James Hilliard]

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

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

We need to use a GAS assembly syntax check so that the assembler
is provided GAS compatible assembly as well.

We can use gasversion to provide a migration path to llvm syntax
for GAS once llvm syntax is natively supported.

Signed-off-by: James Hilliard <james.hilliard1@xxxxxxxxx>
Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
---
Changes v1 -> v2:
  - use gasversion to detect assembly variant
---
 tools/lib/bpf/bpf_helpers.h | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
index 7349b16b8e2f..5b98f5506798 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 +148,18 @@ 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 .gasversion.\n\t"
+		     "mov %%r1,%[ctx]\n\t"
+		     "mov %%r2,%[map]\n\t"
+		     "mov %%r3,%[slot]\n\t"
+		     ".else\n\t"
+		     "r1 = %[ctx]\n\t"
 		     "r2 = %[map]\n\t"
 		     "r3 = %[slot]\n\t"
+		     ".endif\n\t"
 		     "call 12"
 		     :: [ctx]"r"(ctx), [map]"r"(map), [slot]"i"(slot)
 		     : "r0", "r1", "r2", "r3", "r4", "r5");
-- 
2.34.1