On Wed, Jun 26, 2019 at 10:44:47PM -0500, Josh Poimboeuf wrote:
> > > How about the following approach instead? This is the only other way I
> > > can think of to annotate a jump table so that objtool can distinguish
> > > it:
> > >
> > > #define __annotate_jump_table __section(".jump_table.rodata")
> > >
> > > Then bpf would just need the following:
> > >
> > > - static const void *jumptable[256] = {
> > > + static const void __annotate_jump_table *jumptable[256] = {
> > >
> > > This would be less magical and fragile than my original approach.
> > >
> > > I think the jump table would still be placed with all the other rodata,
> > > like before, because the vmlinux linker script recognizes the section
> > > ".rodata" suffix and bundles them all together.
> >
> > I like it if that works :)
> > Definitely cleaner.
> > May be a bit more linker script magic would be necessary,
> > but hopefully not.
> > And no need to rely on gcc style of mangling static vars.
The last patch was based weird, this one's based on upstream. Will test
tomorrow.
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 8aaf7cd026b0..84212bcb5015 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -116,9 +116,14 @@ void ftrace_likely_update(struct ftrace_likely_data *f,
int val,
".pushsection .discard.unreachable\n\t" \
".long 999b - .\n\t" \
".popsection\n\t"
+
+/* Annotate a C jump table to enable objtool to follow the code flow */
+#define __annotate_jump_table __section(".jump_table.rodata")
+
#else
#define annotate_reachable()
#define annotate_unreachable()
+#define __annotate_jump_table
#endif
#ifndef ASM_UNREACHABLE
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 080e2bb644cc..e67977e22967 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -1299,7 +1299,7 @@ static u64 ___bpf_prog_run(u64 *regs, const struct
bpf_insn *insn, u64 *stack)
{
#define BPF_INSN_2_LBL(x, y) [BPF_##x | BPF_##y] = &&x##_##y
#define BPF_INSN_3_LBL(x, y, z) [BPF_##x | BPF_##y | BPF_##z] = &&x##_##y##_##z
- static const void *jumptable[256] = {
+ static const void __annotate_jump_table *jumptable[256] = {
[0 ... 255] = &&default_label,
/* Now overwrite non-defaults ... */
BPF_INSN_MAP(BPF_INSN_2_LBL, BPF_INSN_3_LBL),
@@ -1558,7 +1558,6 @@ static u64 ___bpf_prog_run(u64 *regs, const struct
bpf_insn *insn, u64 *stack)
BUG_ON(1);
return 0;
}
-STACK_FRAME_NON_STANDARD(___bpf_prog_run); /* jump table */
#define PROG_NAME(stack_size) __bpf_prog_run##stack_size
#define DEFINE_BPF_PROG_RUN(stack_size) \
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 172f99195726..6ade2b32f484 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -18,6 +18,8 @@
#define FAKE_JUMP_OFFSET -1
+#define C_JUMP_TABLE_SECTION ".jump_table.rodata"
+
struct alternative {
struct list_head list;
struct instruction *insn;
@@ -1035,9 +1037,15 @@ static struct rela *find_switch_table(struct
objtool_file *file,
/*
* Make sure the .rodata address isn't associated with a
- * symbol. gcc jump tables are anonymous data.
+ * symbol. GCC jump tables are anonymous data.
+ *
+ * Also support C jump tables which are in the same format as
+ * switch jump tables. For objtool to recognize them, they
+ * need to be placed in the C_JUMP_TABLE_SECTION section. They
+ * have symbols associated with them.
*/
- if (find_symbol_containing(rodata_sec, table_offset))
+ if (find_symbol_containing(rodata_sec, table_offset) &&
+ strcmp(rodata_sec->name, C_JUMP_TABLE_SECTION))
continue;
rodata_rela = find_rela_by_dest(rodata_sec, table_offset);
@@ -1277,13 +1285,18 @@ static void mark_rodata(struct objtool_file *file)
bool found = false;
/*
- * This searches for the .rodata section or multiple .rodata.func_name
- * sections if -fdata-sections is being used. The .str.1.1 and .str.1.8
- * rodata sections are ignored as they don't contain jump tables.
+ * Search for the following rodata sections, each of which can
+ * potentially contain jump tables:
+ *
+ * - .rodata: can contain GCC switch tables
+ * - .rodata.<func>: same, if -fdata-sections is being used
+ * - .jump_table.rodata: contains C annotated jump tables
+ *
+ * .rodata.str1.* sections are ignored; they don't contain jump tables.
*/
for_each_sec(file, sec) {
- if (!strncmp(sec->name, ".rodata", 7) &&
- !strstr(sec->name, ".str1.")) {
+ if ((!strncmp(sec->name, ".rodata", 7) && !strstr(sec->name,
".str1.")) ||
+ !strcmp(sec->name, C_JUMP_TABLE_SECTION)) {
sec->rodata = true;
found = true;
}
