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

[Gary Guo <gary@garyguo.net>]

On Wed, 24 Jul 2024 18:14:58 +0200
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` 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@kernel.org>
> ---
> 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;
> +

I wonder if we should use dwarf for this. There's DW_AT_noreturn which
tells us exactly what we want. This would also remove the need for the
hardcoded C symbol list. I do recognise that debug info is not required
for objtool though...

>  		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