Re: [PATCH 3/3] bpf: set index entry for a VAR_DECL in CO-RE relocs

[Cupertino Miranda]

Thanks! Added changelog and Pushed!

On 14-10-2024 19:08, Cupertino Miranda wrote:

On 30-09-2024 18:47, David Faust wrote:

On 9/27/24 09:49, Cupertino Miranda wrote:

CO-RE accesses with non pointer struct variables will also generate a
"0" string access within the CO-RE relocation.
The first index within the access string, has sort of a different
meaning then the remaining of the indexes.
For i0:i1:...:in being an access index for "struct A a" declaration, its
semantics are represented by:
   (&a + (sizeof(struct A) * i0) + offsetof(i1:...:in)

I can guess the answer, but is this semantic actually documented anywhere?

I don't think so. At least I am not aware off.

We may want to see about adding this in the "official" kernel BTF docs
since IMO this special meaning of the first index is not at all obvious >
Patch LGTM.
Thanks.

---
  gcc/config/bpf/core-builtins.cc                  |  5 ++++-
  gcc/testsuite/gcc.target/bpf/core-builtin-1.c    | 16 ++++++++--------
  gcc/testsuite/gcc.target/bpf/core-builtin-2.c    |  3 ++-
  .../gcc.target/bpf/core-builtin-exprlist-1.c     | 16 ++++++++--------
  4 files changed, 22 insertions(+), 18 deletions(-)

diff --git a/gcc/config/bpf/core-builtins.cc b/gcc/config/bpf/core- builtins.cc

index cdfb356660e..fc6379cf028 100644
--- a/gcc/config/bpf/core-builtins.cc
+++ b/gcc/config/bpf/core-builtins.cc

@@ -698,10 +698,13 @@ compute_field_expr (tree node, unsigned int *accessors,

                    access_node, false, callback);
        return n;
+    case VAR_DECL:
+      accessors[0] = 0;
+      return 1;
+
      case ADDR_EXPR:
      case CALL_EXPR:
      case SSA_NAME:
-    case VAR_DECL:
      case PARM_DECL:
        return 0;
      default:

diff --git a/gcc/testsuite/gcc.target/bpf/core-builtin-1.c b/gcc/ testsuite/gcc.target/bpf/core-builtin-1.c

index b4f9998afb8..0706005f0e5 100644
--- a/gcc/testsuite/gcc.target/bpf/core-builtin-1.c
+++ b/gcc/testsuite/gcc.target/bpf/core-builtin-1.c
@@ -24,16 +24,16 @@ unsigned long ula[8];
  unsigned long
  func (void)
  {
-  /* 1 */
+  /* 0:1 */
    int b = _(my_s.b);
-  /* 2 */
+  /* 0:2 */
    char c = _(my_s.c);
-  /* 2:3 */
+  /* 0:2:3 */
    unsigned char uc = _(my_u.uc[3]);
-  /* 6 */
+  /* 0:6 */
    unsigned long ul = _(ula[6]);
    return b + c + uc + ul;
@@ -55,10 +55,10 @@ u_ptr (union U *pu)
    return x;
  }

-/* { dg-final { scan-assembler-times "ascii \"1.0\"\[\t \]+ \[^\n\]*btf_aux_string" 1 } } */ -/* { dg-final { scan-assembler-times "ascii \"2.0\"\[\t \]+ \[^\n\]*btf_aux_string" 1 } } */ -/* { dg-final { scan-assembler-times "ascii \"2:3.0\"\[\t \]+ \[^\n\]*btf_aux_string" 1 } } */ -/* { dg-final { scan-assembler-times "ascii \"6.0\"\[\t \]+ \[^\n\]*btf_aux_string" 1 } } */ +/* { dg-final { scan-assembler-times "ascii \"0:1.0\"\[\t \]+ \[^\n\]*btf_aux_string" 1 } } */ +/* { dg-final { scan-assembler-times "ascii \"0:2.0\"\[\t \]+ \[^\n\]*btf_aux_string" 1 } } */ +/* { dg-final { scan-assembler-times "ascii \"0:2:3.0\"\[\t \]+ \[^\n\]*btf_aux_string" 1 } } */ +/* { dg-final { scan-assembler-times "ascii \"0:6.0\"\[\t \]+ \[^\n\]*btf_aux_string" 1 } } */   /* { dg-final { scan-assembler-times "ascii \"0:2.0\"\[\t \]+ \[^\n\]*btf_aux_string" 1 } } */   /* { dg-final { scan-assembler-times "ascii \"0:2:3.0\"\[\t \]+ \[^\n\]*btf_aux_string" 1 } } */ diff --git a/gcc/testsuite/gcc.target/bpf/core-builtin-2.c b/gcc/ testsuite/gcc.target/bpf/core-builtin-2.c

index b72e2566b71..04b3f6b2652 100644
--- a/gcc/testsuite/gcc.target/bpf/core-builtin-2.c
+++ b/gcc/testsuite/gcc.target/bpf/core-builtin-2.c
@@ -16,11 +16,12 @@ struct S foo;
  void func (void)
  {
+  /* 0:1:3:2 */
    char *x = __builtin_preserve_access_index (&foo.u[3].c);
    *x = 's';
  }

  /* { dg-final { scan-assembler-times "\[\t \]0x4000002\[\t \]+ \[^\n\]*btt_info" 1 } } */ -/* { dg-final { scan-assembler-times "ascii \"1:3:2.0\"\[\t \]+ \[^\n\]*btf_aux_string" 1 } } */ +/* { dg-final { scan-assembler-times "ascii \"0:1:3:2.0\"\[\t \]+ \[^\n\]*btf_aux_string" 1 } } */

  /* { dg-final { scan-assembler-times "bpfcr_type" 1 } } */

diff --git a/gcc/testsuite/gcc.target/bpf/core-builtin-exprlist-1.c b/gcc/testsuite/gcc.target/bpf/core-builtin-exprlist-1.c

index 8ce4a6e70de..c53daf81c5f 100644
--- a/gcc/testsuite/gcc.target/bpf/core-builtin-exprlist-1.c
+++ b/gcc/testsuite/gcc.target/bpf/core-builtin-exprlist-1.c
@@ -31,16 +31,16 @@ func (void)
    int ic;
    __builtin_preserve_access_index (({
-    /* 1 */
+    /* 0:1 */
      b = my_s.b;
-    /* 2 */
+    /* 0:2 */
      ic = my_s.c;
-    /* 2:3 */
+    /* 0:2:3 */
      uc = my_u.uc[3];
-    /* 6 */
+    /* 0:6 */
      ul = ula[6];
    }));
@@ -65,10 +65,10 @@ u_ptr (union U *pu)
    return x;
  }

-/* { dg-final { scan-assembler-times "ascii \"1.0\"\[\t \]+ \[^\n\]*btf_aux_string" 1 } } */ -/* { dg-final { scan-assembler-times "ascii \"2.0\"\[\t \]+ \[^\n\]*btf_aux_string" 1 } } */ -/* { dg-final { scan-assembler-times "ascii \"2:3.0\"\[\t \]+ \[^\n\]*btf_aux_string" 1 } } */ -/* { dg-final { scan-assembler-times "ascii \"6.0\"\[\t \]+ \[^\n\]*btf_aux_string" 1 } } */ +/* { dg-final { scan-assembler-times "ascii \"0:1.0\"\[\t \]+ \[^\n\]*btf_aux_string" 1 } } */ +/* { dg-final { scan-assembler-times "ascii \"0:2.0\"\[\t \]+ \[^\n\]*btf_aux_string" 1 } } */ +/* { dg-final { scan-assembler-times "ascii \"0:2:3.0\"\[\t \]+ \[^\n\]*btf_aux_string" 1 } } */ +/* { dg-final { scan-assembler-times "ascii \"0:6.0\"\[\t \]+ \[^\n\]*btf_aux_string" 1 } } */   /* { dg-final { scan-assembler-times "ascii \"0:2.0\"\[\t \]+ \[^\n\]*btf_aux_string" 1 } } */   /* { dg-final { scan-assembler-times "ascii \"0:2:3.0\"\[\t \]+ \[^\n\]*btf_aux_string" 1 } } */