[PATCH v2 5/6] objtool: list `noreturn` Rust functions

[Miguel Ojeda]

Rust functions may be `noreturn` (i.e. diverging) by returning the
"never" type, `!`, e.g.

    fn f() -> ! {
        loop {}
    }

Thus list the known `noreturn` functions to avoid such warnings.

Without this, `objtool` would complain if enabled for Rust, e.g.:

    rust/core.o: warning: objtool:
    _R...9panic_fmt() falls through to next function _R...18panic_nounwind_fmt()

    rust/alloc.o: warning: objtool:
    .text: unexpected end of section

In order to do so, we cannot match symbols' names exactly, for two
reasons:

  - Rust mangling scheme [1] contains disambiguators [2] which we
    cannot predict (e.g. they may vary depending on the compiler version).

    One possibility to solve this would be to parse v0 and ignore/zero
    those before comparison.

  - Some of the diverging functions come from `core` and `alloc`, i.e.
    the Rust standard library, which may change with each compiler version
    since they are implementation details (e.g. `panic_internals`).

Thus, to workaround both issues, only part of the symbols are matched,
instead of using the `NORETURN` macro in `noreturns.h`.

Ideally, just like for the C side, we should have a better solution. For
instance, the compiler could give us the list via something like:

    $ rustc --print noreturns ...

Link: https://rust-lang.github.io/rfcs/2603-rust-symbol-name-mangling-v0.html [1]
Link: https://doc.rust-lang.org/rustc/symbol-mangling/v0.html#disambiguator [2]
Signed-off-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
---
Please let me know if there is a better solution -- what kind of solution was
being thought about for C as mentioned in `noreturns.h`? Would it help for Rust?

 tools/objtool/check.c     | 36 +++++++++++++++++++++++++++++++++++-
 tools/objtool/noreturns.h |  2 ++
 2 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 0a33d9195b7a..0afdcee038fd 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -177,6 +177,20 @@ static bool is_sibling_call(struct instruction *insn)
 	return (is_static_jump(insn) && insn_call_dest(insn));
 }

+/*
+ * Checks if a string ends with another.
+ */
+static bool str_ends_with(const char *s, const char *sub)
+{
+	const int slen = strlen(s);
+	const int sublen = strlen(sub);
+
+	if (sublen > slen)
+		return 0;
+
+	return !memcmp(s + slen - sublen, sub, sublen);
+}
+
 /*
  * This checks to see if the given function is a "noreturn" function.
  *
@@ -202,10 +216,30 @@ static bool __dead_end_function(struct objtool_file *file, struct symbol *func,
 	if (!func)
 		return false;

-	if (func->bind == STB_GLOBAL || func->bind == STB_WEAK)
+	if (func->bind == STB_GLOBAL || func->bind == STB_WEAK) {
+		/*
+		 * Rust standard library functions.
+		 *
+		 * These are just heuristics -- we do not control the precise symbol
+		 * name, due to the crate disambiguators (which depend on the compiler)
+		 * as well as changes to the source code itself between versions.
+		 */
+		if (!strncmp(func->name, "_R", 2) &&
+		    (str_ends_with(func->name, "_4core6option13unwrap_failed")			||
+		     str_ends_with(func->name, "_4core6result13unwrap_failed")			||
+		     str_ends_with(func->name, "_4core9panicking5panic")			||
+		     str_ends_with(func->name, "_4core9panicking9panic_fmt")			||
+		     str_ends_with(func->name, "_4core9panicking14panic_explicit")		||
+		     str_ends_with(func->name, "_4core9panicking18panic_bounds_check")		||
+		     strstr(func->name, "_4core9panicking11panic_const24panic_const_")		||
+		     (strstr(func->name, "_4core5slice5index24slice_") &&
+		      str_ends_with(func->name, "_fail"))))
+			return true;
+
 		for (i = 0; i < ARRAY_SIZE(global_noreturns); i++)
 			if (!strcmp(func->name, global_noreturns[i]))
 				return true;
+	}

 	if (func->bind == STB_WEAK)
 		return false;
diff --git a/tools/objtool/noreturns.h b/tools/objtool/noreturns.h
index 7ebf29c91184..82a001ac433b 100644
--- a/tools/objtool/noreturns.h
+++ b/tools/objtool/noreturns.h
@@ -35,6 +35,8 @@ NORETURN(panic)
 NORETURN(panic_smp_self_stop)
 NORETURN(rest_init)
 NORETURN(rewind_stack_and_make_dead)
+NORETURN(rust_begin_unwind)
+NORETURN(rust_helper_BUG)
 NORETURN(sev_es_terminate)
 NORETURN(snp_abort)
 NORETURN(start_kernel)
--
2.45.2