Commit 14e5ce79 authored by James Hilliard's avatar James Hilliard Committed by Daniel Borkmann
Browse files

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 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: default avatarJames Hilliard <james.hilliard1@gmail.com>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220829210546.755377-1-james.hilliard1@gmail.com
parent 6f95de6d

tools/lib/bpf/bpf_helpers.h

+13 −6
Original line number Diff line number Diff line
@@ -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");