[PATCH dwarves] btf_encoder: verify 0 address DWARF variables are really in ELF section

[Alan Maguire <alan.maguire@oracle.com>]

We use the DWARF location information to match a variable with its
associated ELF section.  In the case of per-CPU variables their
ELF section address range starts at 0, so any 0 address variables will
appear to belong in that ELF section.  However, for "discard" sections
DWARF encodes the associated variables with address location 0 so
we need to double-check that address 0 variables really are in the
associated section by checking the ELF symbol table.

This resolves an issue exposed by CONFIG_DEBUG_FORCE_WEAK_PER_CPU=y
kernel builds where __pcpu_* dummary variables in a .discard section
get misclassified as belonging in the per-CPU variable section since
they specify location address 0.

Reported-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
---
 btf_encoder.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/btf_encoder.c b/btf_encoder.c
index 3754884..04f547c 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -2189,6 +2189,26 @@ static bool filter_variable_name(const char *name)
 	return false;
 }
 
+bool variable_in_sec(struct btf_encoder *encoder, const char *name, size_t shndx)
+{
+	uint32_t sym_sec_idx;
+	uint32_t core_id;
+	GElf_Sym sym;
+
+	elf_symtab__for_each_symbol_index(encoder->symtab, core_id, sym, sym_sec_idx) {
+		const char *sym_name;
+
+		if (sym_sec_idx != shndx || elf_sym__type(&sym) != STT_OBJECT)
+			continue;
+		sym_name = elf_sym__name(&sym, encoder->symtab);
+		if (!sym_name)
+			continue;
+		if (strcmp(name, sym_name) == 0)
+			return true;
+	}
+	return false;
+}
+
 static int btf_encoder__encode_cu_variables(struct btf_encoder *encoder)
 {
 	struct cu *cu = encoder->cu;
@@ -2258,6 +2278,13 @@ static int btf_encoder__encode_cu_variables(struct btf_encoder *encoder)
 		if (filter_variable_name(name))
 			continue;
 
+		/* A 0 address may be in a "discard" section; DWARF provides
+		 * location information with address 0 for such variables.
+		 * Ensure the variable really is in this section by checking
+		 * the ELF symtab.
+		 */
+		if (addr == 0 && !variable_in_sec(encoder, name, shndx))
+			continue;
 		/* Check for invalid BTF names */
 		if (!btf_name_valid(name)) {
 			dump_invalid_symbol("Found invalid variable name when encoding btf",
-- 
2.31.1