[PATCH dwarves 2/2] btf_encoder: fix skipping per-CPU variables at offset 0

[Andrii Nakryiko <andrii@kernel.org>]

Adjust pahole logic of skipping any per-CPU symbol with offset 0, which is
especially bad for kernel modules, because it most certainly skips the very
first per-CPU variable.

Instead, do collect per-CPU ELF symbol with 0 offset, but do extra check for
non-kernel module case by verifying that ELF symbol name and DWARF variable
name match. Due to the bug of DWARF name of variable sometimes being NULL,
this is necessarily too pessimistic check (e.g., on my vmlinux image,
fixed_percpu_data variable is still not emitted due to missing DWARF variable
name), it allows to emit data for all module per-CPU variables.

Fixes: f3d9054ba8ff ("btf_encoder: Teach pahole to store percpu variables in vmlinux BTF.")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
 btf_encoder.c | 40 ++++++++++++++++++++++++----------------
 1 file changed, 24 insertions(+), 16 deletions(-)

diff --git a/btf_encoder.c b/btf_encoder.c
index a7d484765ce2..1d7817078f89 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -412,21 +412,6 @@ static int collect_percpu_var(struct btf_elf *btfe, GElf_Sym *sym)
 		return 0;
 
 	addr = elf_sym__value(sym);
-	/*
-	 * Store only those symbols that have allocated space in the percpu section.
-	 * This excludes the following three types of symbols:
-	 *
-	 *  1. __ADDRESSABLE(sym), which are forcely emitted as symbols.
-	 *  2. __UNIQUE_ID(prefix), which are introduced to generate unique ids.
-	 *  3. __exitcall(fn), functions which are labeled as exit calls.
-	 *
-	 * In addition, the variables defined using DEFINE_PERCPU_FIRST are
-	 * also not included, which currently includes:
-	 *
-	 *  1. fixed_percpu_data
-	 */
-	if (!addr)
-		return 0;
 
 	size = elf_sym__size(sym);
 	if (!size)
@@ -652,7 +637,7 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
 
 	cu__for_each_variable(cu, core_id, pos) {
 		uint32_t size, type, linkage;
-		const char *name;
+		const char *name, *dwarf_name;
 		uint64_t addr;
 		int id;
 
@@ -680,6 +665,29 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
 		if (!percpu_var_exists(addr, &size, &name))
 			continue; /* not a per-CPU variable */
 
+		/* A lot of "special" DWARF variables (e.g, __UNIQUE_ID___xxx)
+		 * have addr == 0, which is the same as, say, valid
+		 * fixed_percpu_data per-CPU variable. To distinguish between
+		 * them, additionally compare DWARF and ELF symbol names. If
+		 * DWARF doesn't provide proper name, pessimistically assume
+		 * bad variable.
+		 *
+		 * Examples of such special variables are:
+		 *
+		 *  1. __ADDRESSABLE(sym), which are forcely emitted as symbols.
+		 *  2. __UNIQUE_ID(prefix), which are introduced to generate unique ids.
+		 *  3. __exitcall(fn), functions which are labeled as exit calls.
+		 *
+		 *  This is relevant only for vmlinux image, as for kernel
+		 *  modules per-CPU data section has non-zero offset so all
+		 *  per-CPU symbols have non-zero values.
+		 */
+		if (var->ip.addr == 0) {
+			dwarf_name = variable__name(var, cu);
+			if (!dwarf_name || strcmp(dwarf_name, name))
+				continue;
+		}
+
 		if (var->spec)
 			var = var->spec;
 
-- 
2.24.1