linux-kernel - Re: [PATCH v7 15/22] objtool: Add action to check for absence of absolute relocations

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Message-ID: <87cy6rf8bu.fsf@igel.home>
Date: Mon, 13 Oct 2025 11:40:37 +0200
From: Andreas Schwab <schwab@...ux-m68k.org>
To: Ard Biesheuvel <ardb+git@...gle.com>
Cc: linux-kernel@...r.kernel.org,  linux-efi@...r.kernel.org,
  x86@...nel.org,  Ard Biesheuvel <ardb@...nel.org>,  Borislav Petkov
 <bp@...en8.de>,  Ingo Molnar <mingo@...nel.org>,  Kevin Loughlin
 <kevinloughlin@...gle.com>,  Tom Lendacky <thomas.lendacky@....com>,  Josh
 Poimboeuf <jpoimboe@...nel.org>,  Peter Zijlstra <peterz@...radead.org>,
  Nikunj A Dadhania <nikunj@....com>
Subject: Re: [PATCH v7 15/22] objtool: Add action to check for absence of
 absolute relocations

On Aug 28 2025, Ard Biesheuvel wrote:

> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> index 79eab61cd944..aeefc749e237 100644
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -4686,6 +4686,47 @@ static void disas_warned_funcs(struct objtool_file *file)
>  		disas_funcs(funcs);
>  }
>  
> +__weak bool arch_absolute_reloc(struct elf *elf, struct reloc *reloc)
> +{
> +	unsigned int type = reloc_type(reloc);
> +	size_t sz = elf_addr_size(elf);
> +
> +	return (sz == 8) ? (type == R_ABS64) : (type == R_ABS32);
> +}
> +
> +static int check_abs_references(struct objtool_file *file)
> +{
> +	struct section *sec;
> +	struct reloc *reloc;
> +	int ret = 0;
> +
> +	for_each_sec(file, sec) {
> +		/* absolute references in non-loadable sections are fine */
> +		if (!(sec->sh.sh_flags & SHF_ALLOC))
> +			continue;
> +
> +		/* section must have an associated .rela section */
> +		if (!sec->rsec)
> +			continue;
> +
> +		/*
> +		 * Special case for compiler generated metadata that is not
> +		 * consumed until after boot.
> +		 */
> +		if (!strcmp(sec->name, "__patchable_function_entries"))
> +			continue;
> +
> +		for_each_reloc(sec->rsec, reloc) {
> +			if (arch_absolute_reloc(file->elf, reloc)) {
> +				WARN("section %s has absolute relocation at offset 0x%lx",
> +				     sec->name, reloc_offset(reloc));

This is wrong for a 32-bit host:

In file included from check.c:16:
check.c: In function ‘check_abs_references’:
/home/andreas/src/linux/linux-6.18-rc1/tools/objtool/include/objtool/warn.h:47:3: error: format ‘%lx’ expects argument of type ‘long unsigned int’, but argument 7 has type ‘u64’ {aka ‘long long unsigned int’} [-Werror=format=]
   47 |   "%s%s%s: objtool" extra ": " format "\n",  \
      |   ^~~~~~~~~~~~~~~~~
/home/andreas/src/linux/linux-6.18-rc1/tools/objtool/include/objtool/warn.h:54:2: note: in expansion of macro ‘___WARN’
   54 |  ___WARN(severity, "", format, ##__VA_ARGS__)
      |  ^~~~~~~
/home/andreas/src/linux/linux-6.18-rc1/tools/objtool/include/objtool/warn.h:74:27: note: in expansion of macro ‘__WARN’
   74 | #define WARN(format, ...) __WARN(WARN_STR, format, ##__VA_ARGS__)
      |                           ^~~~~~
check.c:4713:5: note: in expansion of macro ‘WARN’
 4713 |     WARN("section %s has absolute relocation at offset 0x%lx",
      |     ^~~~
cc1: all warnings being treated as errors

-- 
Andreas Schwab, schwab@...ux-m68k.org
GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510  2552 DF73 E780 A9DA AEC1
"And now for something completely different."