[PATCH bpf-next v3 10/13] bpftool: Emit __bpf_static_offset on verifier-rewritten context types

[Ihor Solodrai <ihor.solodrai@linux.dev>]

The verifier translates accesses to some UAPI context and socket types
into accesses to kernel structures. These accesses require an
unmodified base and an immediate field offset.

Leaving preserve_access_index (PAI) off does not stop the compiler
from merging loads at different field offsets into one load through an
adjusted pointer. The verifier rejects the resulting program.

preserve_static_offset (PSO), available since Clang 18, preserves
static field offsets in load and store instruction immediates. Apply
it to the 13 types that need static offsets in vmlinux.h through
__bpf_static_offset.

Eduard introduced PSO in Clang and proposed annotating BPF context
types in 2023 [1]. His v3 RFC [2] left pt_regs annotations in
architecture headers unresolved. However, since commit
de6c7d99f898 ("bpf: Relax fixed offset check for PTR_TO_CTX"), program
types without convert_ctx_access() accept fixed offsets from modified
context pointers. pt_regs can now keep PAI, while PSO on
bpf_perf_event_data covers its regs member.

PSO creates no CO-RE relocations, so BPF_NO_PRESERVE_ACCESS_INDEX does
not disable it.

Clang 18 through 23 crash on __builtin_preserve_field_info over a PSO
record, including bpf_core_field_*() and the CO-RE bitfield helpers.
-D__bpf_static_offset= can disable PSO for affected programs. The
crash has already been forward fixed in LLVM [3].

[1]: https://lore.kernel.org/bpf/20231208000531.19179-1-eddyz87@gmail.com/
[2]: https://lore.kernel.org/bpf/20231220133411.22978-1-eddyz87@gmail.com/
[3]: https://github.com/llvm/llvm-project/pull/234280

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
---
 tools/bpf/bpftool/btf.c                          | 16 ++++++++++++++--
 .../bpf/bpftool_btf_dump_sorted.expected         |  8 ++++++++
 .../bpf/bpftool_btf_dump_unsorted.expected       |  8 ++++++++
 3 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
index 86ab844898db..03da4f6821f1 100644
--- a/tools/bpf/bpftool/btf.c
+++ b/tools/bpf/bpftool/btf.c
@@ -1204,12 +1204,17 @@ static int btf_c_annotate_records(struct btf *btf)
 
 	for (i = 1; i < cnt; i++) {
 		const struct btf_type *t = btf__type_by_id(btf, i);
+		const char *attr;
 
 		if (!btf_is_composite(t))
 			continue;
-		if (non_relocatable[i])
+		if (!non_relocatable[i])
+			attr = "__bpf_relocatable";
+		else if (btf_c_needs_static_offset(btf, t))
+			attr = "__bpf_static_offset";
+		else
 			continue;
-		err = btf__add_decl_attr(btf, "__bpf_relocatable", i, -1);
+		err = btf__add_decl_attr(btf, attr, i, -1);
 		if (err < 0)
 			goto out;
 	}
@@ -1262,6 +1267,13 @@ static int dump_btf_c(struct btf *btf,
 	printf("#define __bpf_relocatable\n");
 	printf("#endif\n");
 	printf("#endif\n\n");
+	printf("#ifndef __bpf_static_offset\n");
+	printf("#if __has_attribute(preserve_static_offset)\n");
+	printf("#define __bpf_static_offset __attribute__((preserve_static_offset))\n");
+	printf("#else\n");
+	printf("#define __bpf_static_offset\n");
+	printf("#endif\n");
+	printf("#endif\n\n");
 	printf("#ifndef __ksym\n");
 	printf("#define __ksym __attribute__((section(\".ksyms\")))\n");
 	printf("#endif\n\n");
diff --git a/tools/testing/selftests/bpf/bpftool_btf_dump_sorted.expected b/tools/testing/selftests/bpf/bpftool_btf_dump_sorted.expected
index f2389e3dceed..b56b676b96a5 100644
--- a/tools/testing/selftests/bpf/bpftool_btf_dump_sorted.expected
+++ b/tools/testing/selftests/bpf/bpftool_btf_dump_sorted.expected
@@ -9,6 +9,14 @@
 #endif
 #endif
 
+#ifndef __bpf_static_offset
+#if __has_attribute(preserve_static_offset)
+#define __bpf_static_offset __attribute__((preserve_static_offset))
+#else
+#define __bpf_static_offset
+#endif
+#endif
+
 #ifndef __ksym
 #define __ksym __attribute__((section(".ksyms")))
 #endif
diff --git a/tools/testing/selftests/bpf/bpftool_btf_dump_unsorted.expected b/tools/testing/selftests/bpf/bpftool_btf_dump_unsorted.expected
index 549045677a45..543db89078af 100644
--- a/tools/testing/selftests/bpf/bpftool_btf_dump_unsorted.expected
+++ b/tools/testing/selftests/bpf/bpftool_btf_dump_unsorted.expected
@@ -9,6 +9,14 @@
 #endif
 #endif
 
+#ifndef __bpf_static_offset
+#if __has_attribute(preserve_static_offset)
+#define __bpf_static_offset __attribute__((preserve_static_offset))
+#else
+#define __bpf_static_offset
+#endif
+#endif
+
 #ifndef __ksym
 #define __ksym __attribute__((section(".ksyms")))
 #endif
-- 
2.56.0