[PATCH bpf-next v1 5/8] bpftool: Emit __bpf_core_pso 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 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, available since Clang 18, preserves static
field offsets in load and store instruction immediates. Apply it to
the 13 types that need static offsets through __bpf_core_pso.

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_core_pso= can disable PSO for affected programs.

[1]: https://lore.kernel.org/bpf/20231208000531.19179-1-eddyz87@gmail.com/
[2]: https://lore.kernel.org/bpf/20231220133411.22978-1-eddyz87@gmail.com/

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 30f30314ec29..fff2ea0a5c7e 100644
--- a/tools/bpf/bpftool/btf.c
+++ b/tools/bpf/bpftool/btf.c
@@ -1203,12 +1203,17 @@ 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 (test_bit(i, no_pai))
+		if (!test_bit(i, no_pai))
+			attr = "__bpf_core_pai";
+		else if (btf_c_needs_static_offset(btf, t))
+			attr = "__bpf_core_pso";
+		else
 			continue;
-		err = btf__add_decl_attr(btf, "__bpf_core_pai", i, -1);
+		err = btf__add_decl_attr(btf, attr, i, -1);
 		if (err < 0)
 			goto out;
 	}
@@ -1251,6 +1256,13 @@ static int dump_btf_c(const struct btf *btf,
 	printf("#define __bpf_core_pai\n");
 	printf("#endif\n");
 	printf("#endif\n\n");
+	printf("#ifndef __bpf_core_pso\n");
+	printf("#if __has_attribute(preserve_static_offset)\n");
+	printf("#define __bpf_core_pso preserve_static_offset\n");
+	printf("#else\n");
+	printf("#define __bpf_core_pso\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 02c8fe8b4859..7da4df1da207 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_core_pso
+#if __has_attribute(preserve_static_offset)
+#define __bpf_core_pso preserve_static_offset
+#else
+#define __bpf_core_pso
+#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 9b6589d2bda3..f082da51d630 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_core_pso
+#if __has_attribute(preserve_static_offset)
+#define __bpf_core_pso preserve_static_offset
+#else
+#define __bpf_core_pso
+#endif
+#endif
+
 #ifndef __ksym
 #define __ksym __attribute__((section(".ksyms")))
 #endif
-- 
2.56.0