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Date: Wed, 26 Jun 2019 22:44:47 -0500
From: Josh Poimboeuf <jpoimboe@...hat.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
Peter Zijlstra <peterz@...radead.org>,
Song Liu <songliubraving@...com>,
Kairui Song <kasong@...hat.com>,
Steven Rostedt <rostedt@...dmis.org>,
Thomas Gleixner <tglx@...utronix.de>,
Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v3 2/4] objtool: Add support for C jump tables
On Wed, Jun 26, 2019 at 07:54:08PM -0700, Alexei Starovoitov wrote:
> On Wed, Jun 26, 2019 at 7:47 PM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> >
> > On Wed, Jun 26, 2019 at 06:42:40PM -0700, Alexei Starovoitov wrote:
> > > > @@ -1035,9 +1038,18 @@ 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. Each jump table should be a static
> > > > + * local const array named "jump_table" for objtool to
> > > > + * recognize it.
> > >
> > > Nacked-by: Alexei Starovoitov <ast@...nel.org>
> > >
> > > It's not acceptable for objtool to dictate kernel naming convention.
> >
> > Abrasive nack notwithstanding, I agree it's not ideal.
> >
> > 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.
Completely untested swag:
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 8341c2fff14f..d6d7dd65a195 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -18,7 +18,7 @@
#define FAKE_JUMP_OFFSET -1
-#define JUMP_TABLE_SYM_PREFIX "jump_table."
+#define C_JUMP_TABLE_SECTION ".jump_table.rodata"
struct alternative {
struct list_head list;
@@ -999,7 +999,6 @@ static struct rela *find_switch_table(struct objtool_file *file,
struct instruction *orig_insn = insn;
struct section *rodata_sec;
unsigned long table_offset;
- struct symbol *sym;
/*
* Backward search using the @first_jump_src links, these help avoid
@@ -1041,15 +1040,12 @@ static struct rela *find_switch_table(struct objtool_file *file,
* symbol. GCC jump tables are anonymous data.
*
* Also support C jump tables which are in the same format as
- * switch jump tables. Each jump table should be a static
- * local const array named "jump_table" for objtool to
- * recognize it. Note: GCC will add a numbered suffix to the
- * ELF symbol name, like "jump_table.12345", which it does for
- * all static local variables.
+ * 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.
*/
- sym = find_symbol_containing(rodata_sec, table_offset);
- if (sym && strncmp(sym->name, JUMP_TABLE_SYM_PREFIX,
- strlen(JUMP_TABLE_SYM_PREFIX)))
+ 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);
@@ -1289,13 +1285,19 @@ 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.
+ * This searches for the following rodata sections, each of which can
+ * potentially contain jump tables:
+ *
+ * - .rodata - can contain GCC switch tables
+ * - .rodata.func_name - if -fdata-sections * is being used
+ * - .jump_table.rodata - contains C annotated jump tables
+ *
+ * The .rodata.str.* sections are ignored because 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;
}
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