Re: [PATCH v2 23/23] objtool, kcsan: Remove memory barrier instrumentation from noinstr

Marco Elver
2021-11-23T11:29:51Z

On Fri, Nov 19, 2021 at 12:31PM -0800, Josh Poimboeuf wrote:
> On Thu, Nov 18, 2021 at 09:10:27AM +0100, Marco Elver wrote:
[...]
> > + if (insn->sec->noinstr && sym->removable_instr) {
[...]
> I'd love to have a clearer name than 'removable_instr', though I'm
> having trouble coming up with something.
[...]

I now have the below as v3 of this patch. The naming isn't entirely
obvious, but coming up with a short name for this is tricky, but
hopefully the comments make it clear. We can of course still pick
another name.

Does that look reasonable?

Note, I'd like this series to sit in -next for a while (probably from
some time next week after sending v3 if there are no further
complaints). By default everything will be picked up by the -rcu tree,
and we're targeting Linux 5.18.

If you feel there might be objtool conflicts coming, this patch could be
taken through another tree as there are no hard dependencies, as long as
this patch reaches mainline before or with the rest.

Thanks,
-- Marco

------ >8 ------

From: Marco Elver <elver@google.com>
Date: Mon, 9 Aug 2021 12:11:14 +0200
Subject: [PATCH] objtool, kcsan: Remove memory barrier instrumentation from
noinstr

Teach objtool to turn instrumentation required for memory barrier
modeling into nops in noinstr text.

The __tsan_func_entry/exit calls are still emitted by compilers even
with the __no_sanitize_thread attribute. The memory barrier
instrumentation will be inserted explicitly (without compiler help), and
thus needs to also explicitly be removed.

Signed-off-by: Marco Elver <elver@google.com>
---
v3:
* s/removable_instr/profiling_func/ (suggested by Josh Poimboeuf)
* Fix and add more comments.

v2:
* Rewrite after rebase to v5.16-rc1.
---
tools/objtool/check.c | 41 ++++++++++++++++++++++++-----
tools/objtool/include/objtool/elf.h | 2 +-
2 files changed, 36 insertions(+), 7 deletions(-)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 61dfb66b30b6..a78186c583f4 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1072,11 +1072,11 @@ static void annotate_call_site(struct objtool_file *file,
}

/*
- * Many compilers cannot disable KCOV with a function attribute
- * so they need a little help, NOP out any KCOV calls from noinstr
- * text.
+ * Many compilers cannot disable KCOV or sanitizer calls with a function
+ * attribute so they need a little help, NOP out any such calls from
+ * noinstr text.
*/
- if (insn->sec->noinstr && sym->kcov) {
+ if (insn->sec->noinstr && sym->profiling_func) {
if (reloc) {
reloc->type = R_NONE;
elf_write_reloc(file->elf, reloc);
@@ -1991,6 +1991,35 @@ static int read_intra_function_calls(struct objtool_file *file)
return 0;
}

+/*
+ * Return true if name matches an instrumentation function, where calls to that
+ * function from noinstr code can safely be removed, but compilers won't do so.
+ */
+static bool is_profiling_func(const char *name)
+{
+ /*
+ * Many compilers cannot disable KCOV with a function attribute.
+ */
+ if (!strncmp(name, "__sanitizer_cov_", 16))
+ return true;
+
+ /*
+ * Compilers currently do not remove __tsan_func_entry/exit with the
+ * __no_sanitize_thread attribute, remove them.
+ *
+ * Memory barrier instrumentation is not emitted by the compiler, but
+ * inserted explicitly, so we need to also remove them.
+ */
+ if (!strncmp(name, "__tsan_func_", 12) ||
+ !strcmp(name, "__kcsan_mb") ||
+ !strcmp(name, "__kcsan_wmb") ||
+ !strcmp(name, "__kcsan_rmb") ||
+ !strcmp(name, "__kcsan_release"))
+ return true;
+
+ return false;
+}
+
static int classify_symbols(struct objtool_file *file)
{
struct section *sec;
@@ -2011,8 +2040,8 @@ static int classify_symbols(struct objtool_file *file)
if (!strcmp(func->name, "__fentry__"))
func->fentry = true;

- if (!strncmp(func->name, "__sanitizer_cov_", 16))
- func->kcov = true;
+ if (is_profiling_func(func->name))
+ func->profiling_func = true;
}
}

diff --git a/tools/objtool/include/objtool/elf.h b/tools/objtool/include/objtool/elf.h
index cdc739fa9a6f..d22336781401 100644
--- a/tools/objtool/include/objtool/elf.h
+++ b/tools/objtool/include/objtool/elf.h
@@ -58,7 +58,7 @@ struct symbol {
u8 static_call_tramp : 1;
u8 retpoline_thunk : 1;
u8 fentry : 1;
- u8 kcov : 1;
+ u8 profiling_func : 1;
struct list_head pv_target;
};

--
2.34.0.rc2.393.gf8c9666880-goog