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`, 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 --emit=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@kernel.org>
---
tools/objtool/check.c | 48 ++++++++++++++++++++++++++++++++++++++-
tools/objtool/noreturns.h | 2 ++
2 files changed, 49 insertions(+), 1 deletion(-)
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 0a33d9195b7a..deace6fca2ed 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -177,6 +177,48 @@ 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);
+}
+
+/*
+ * Checks if a function is a Rust "noreturn" one.
+ */
+static bool is_rust_noreturn(const struct symbol *func)
+{
+ /*
+ * If it does not start with "_R", then it is not a Rust symbol.
+ */
+ if (strncmp(func->name, "_R", 2))
+ return false;
+
+ /*
+ * 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 (since
+ * these come from the Rust standard library).
+ */
+ return 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"));
+}
+
/*
* This checks to see if the given function is a "noreturn" function.
*
@@ -202,10 +244,14 @@ 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) {
+ if (is_rust_noreturn(func))
+ 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
On Thu, Jul 25, 2024 at 08:33:22PM +0200, Miguel Ojeda wrote: > 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`, 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 --emit=noreturns ... Yeah, having added noreturns to objtool myself a few times, it'd be nice to have a way to make these manual lists go away some day. > > 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@kernel.org> Reviewed-by: Kees Cook <kees@kernel.org> -- Kees Cook
On Tue, Aug 06, 2024 at 12:42:41PM -0700, Kees Cook wrote: > On Thu, Jul 25, 2024 at 08:33:22PM +0200, Miguel Ojeda wrote: > > 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`, 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 --emit=noreturns ... > > Yeah, having added noreturns to objtool myself a few times, it'd be nice > to have a way to make these manual lists go away some day. So it would be fairly simple to make objtool consume a magic section emitted by the compiler.. I think we've asked the compiler folks for that at some point even, but I don't have clear recollections.
On Tue, Aug 6, 2024 at 10:22 PM Peter Zijlstra <peterz@infradead.org> wrote: > > So it would be fairly simple to make objtool consume a magic section > emitted by the compiler.. I think we've asked the compiler folks for > that at some point even, but I don't have clear recollections. The section sounds like a good approach -- we will ask the Rust team about it. Then perhaps we can get Clang/GCC to implement something similar too -- for this sort of thing we can use the shorter cycles of `rustc` (and their unstable features concept too) to experiment with these things :) I have also added it to our `rustc` sublist of things we need. Cheers, Miguel
On Thu, Jul 25, 2024 at 8:35 PM Miguel Ojeda <ojeda@kernel.org> wrote: > > 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`, 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 --emit=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@kernel.org> Tested-by: Alice Ryhl <aliceryhl@google.com>
On Thu, Jul 25, 2024 at 08:33:22PM +0200, Miguel Ojeda wrote: > 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`, 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 --emit=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@kernel.org> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> > --- > tools/objtool/check.c | 48 ++++++++++++++++++++++++++++++++++++++- > tools/objtool/noreturns.h | 2 ++ > 2 files changed, 49 insertions(+), 1 deletion(-) > > diff --git a/tools/objtool/check.c b/tools/objtool/check.c > index 0a33d9195b7a..deace6fca2ed 100644 > --- a/tools/objtool/check.c > +++ b/tools/objtool/check.c > @@ -177,6 +177,48 @@ 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); > +} > + > +/* > + * Checks if a function is a Rust "noreturn" one. > + */ > +static bool is_rust_noreturn(const struct symbol *func) > +{ > + /* > + * If it does not start with "_R", then it is not a Rust symbol. > + */ > + if (strncmp(func->name, "_R", 2)) > + return false; > + > + /* > + * 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 (since > + * these come from the Rust standard library). > + */ > + return 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")); > +} > + > /* > * This checks to see if the given function is a "noreturn" function. > * > @@ -202,10 +244,14 @@ 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) { > + if (is_rust_noreturn(func)) > + 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 >
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