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

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On 9/9/22 5:26 PM, Andrii Nakryiko wrote:
On Fri, Sep 9, 2022 at 3:46 PM James Hilliard <james.hilliard1@xxxxxxxxx> wrote:

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.

Signed-off-by: James Hilliard <james.hilliard1@xxxxxxxxx>
Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
---
  tools/lib/bpf/bpf_helpers.h | 10 +++++-----
  1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
index 7349b16b8e2f..30fc95e7cd76 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
@@ -148,8 +148,8 @@ 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"
                      "r2 = %[map]\n\t"

will this compile as is on GCC-BPF? I'm trying to understand what's
the point. Once GCC supports this ASM syntax we can add similar check
to __clang_major__, instead of allowing it for all GCC versions?

We must have done __clang_major__ check for a reason, old Clangs
probably had some problems compiling this. Maybe Daniel remembers?

Yes, clang >= 8 is needed to ensure 'slot' is propagated as 'const' instead of variables. clang 6 added support for inline asm but clang 8
is needed for above 'const' propagation into asm code.


--
2.34.1




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