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libbpf: Add GCC support for bpf_tail_call_static

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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>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220829210546.755377-1-james.hilliard1@gmail.com
This commit is contained in:
James Hilliard 2022-08-29 15:05:46 -06:00 committed by Daniel Borkmann
parent 6f95de6d71
commit 14e5ce7994
1 changed files with 13 additions and 6 deletions
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19
tools/lib/bpf/bpf_helpers.h
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@ -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");