linux-kernel - [PATCH] libbpf: add GCC support for bpf_tail_call

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Message-Id: <20220829195349.706672-1-james.hilliard1@gmail.com>
Date:   Mon, 29 Aug 2022 13:53:49 -0600
From:   James Hilliard <james.hilliard1@...il.com>
To:     bpf@...r.kernel.org
Cc:     James Hilliard <james.hilliard1@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: [PATCH] libbpf: add GCC support for bpf_tail_call_static

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 also need to check for the GCC style __BPF__ in addition to __bpf__
for this to work as GCC does not define __bpf__.

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@...il.com>
---
 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..a0650b840cda 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__) || 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