linux-kernel - Re: Linux 4.16-rc4

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Message-ID: <871sgyfnh2.fsf@turtle.gmx.de>
Date:   Mon, 05 Mar 2018 18:20:41 +0100
From:   Sven Joachim <svenjoac@....de>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: Linux 4.16-rc4

On 2018-03-05 07:38 -0600, Josh Poimboeuf wrote:

> On Mon, Mar 05, 2018 at 11:17:48AM +0100, Peter Zijlstra wrote:
>> On Mon, Mar 05, 2018 at 09:09:31AM +0100, Sven Joachim wrote:
>> > On 2018-03-04 15:15 -0800, Linus Torvalds wrote:
>> > 
>> > > Hmm. A reasonably calm week - the biggest change is to the 'kvm-stat'
>> > > tool, not any actual kernel files.
>> > >
>> > > But there's small changes all over, with architecture updates (x86,
>> > > s390, arm, parisc) and drivers (media, md, gpu, sound) being the bulk
>> > > of it.  But there's some filesystem fixes (mostly btrfs),
>> > > documentation updates etc too.
>> > >
>> > > Go test,
>> > 
>> > Huh, this version does not build for me:
>> > 
>> > ,----
>> > |   CALL    scripts/checksyscalls.sh
>> > |   DESCEND  objtool
>> > |   CC       /usr/local/src/linux/tools/objtool/check.o
>> > | In file included from check.c:26:0:
>> > | check.c: In function 'read_retpoline_hints':
>> > | warn.h:57:3: error: format '%ld' expects argument of type 'long int', but argument 5 has type 'unsigned int' [-Werror=format=]
>> > |    "%s: warning: objtool: " format "\n", \
>> > |    ^
>> > | check.c:1135:3: note: in expansion of macro 'WARN'
>> > |    WARN("retpoline_safe size mismatch: %d %ld", sec->len, sizeof(unsigned long));
>> > |    ^~~~
>> > | check.c:1135:44: note: format string is defined here
>> > |    WARN("retpoline_safe size mismatch: %d %ld", sec->len, sizeof(unsigned long));
>> > |                                           ~~^
>> > |                                           %d
>> > | cc1: all warnings being treated as errors
>> > | mv: cannot stat '/usr/local/src/linux/tools/objtool/.check.o.tmp': No such file or directory
>> > | /usr/local/src/linux/tools/build/Makefile.build:96: recipe for target '/usr/local/src/linux/tools/objtool/check.o' failed
>> > | make[3]: *** [/usr/local/src/linux/tools/objtool/check.o] Error 1
>> > `----
>> > 
>> > This might be because I still use a 32-bit userland with a 64-bit
>> > kernel.
>> 
>> Urgh, so sizeof() returns size_t which is confusing. But what is the
>> actual value of sizeof(unsigned long) for you? I suspect cross building
>> objtool doesn't work right at all. We build the kernel using LP64, and
>> its retpoline_safe section is 8 bytes. But if we build objtool as ILP32
>> then it would interpret things as 4 bytes.
>> 
>> Josh, is that supposed to work? I could of course move the retpoline
>> annotation over to 4 byte relative addressing which would fix this one
>> issue. Is that really the only case?
>
> I suspect it may be the only case.  In most cases objtool relies on
> libelf for handling the object bit width.
>
> It looks like read_retpoline_hints() is "special" compared to the other
> annotation reading functions.  The easiest fix would be to convert it to
> be like the others.
>
> Sven, can you test this patch?
>
> ---
>
> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> index 472e64e95891..e00ff29cb7ea 100644
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -1112,42 +1112,29 @@ static int read_unwind_hints(struct objtool_file *file)
>  
>  static int read_retpoline_hints(struct objtool_file *file)
>  {
> -	struct section *sec, *relasec;
> +	struct section *sec;
>  	struct instruction *insn;
>  	struct rela *rela;
> -	int i;
>  
> -	sec = find_section_by_name(file->elf, ".discard.retpoline_safe");
> +	sec = find_section_by_name(file->elf, ".rela.discard.retpoline_safe");
>  	if (!sec)
>  		return 0;
>  
> -	relasec = sec->rela;
> -	if (!relasec) {
> -		WARN("missing .rela.discard.retpoline_safe section");
> -		return -1;
> -	}
> -
> -	if (sec->len % sizeof(unsigned long)) {
> -		WARN("retpoline_safe size mismatch: %d %ld", sec->len, sizeof(unsigned long));
> -		return -1;
> -	}
> -
> -	for (i = 0; i < sec->len / sizeof(unsigned long); i++) {
> -		rela = find_rela_by_dest(sec, i * sizeof(unsigned long));
> -		if (!rela) {
> -			WARN("can't find rela for retpoline_safe[%d]", i);
> +	list_for_each_entry(rela, &sec->rela_list, list) {
> +		if (rela->sym->type != STT_SECTION) {
> +			WARN("unexpected relocation symbol type in %s", sec->name);
>  			return -1;
>  		}
>  
>  		insn = find_insn(file, rela->sym->sec, rela->addend);
>  		if (!insn) {
> -			WARN("can't find insn for retpoline_safe[%d]", i);
> +			WARN("bad .discard.retpoline_safe entry");
>  			return -1;
>  		}
>  
>  		if (insn->type != INSN_JUMP_DYNAMIC &&
>  		    insn->type != INSN_CALL_DYNAMIC) {
> -			WARN_FUNC("retpoline_safe hint not a indirect jump/call",
> +			WARN_FUNC("retpoline_safe hint not an indirect jump/call",
>  				  insn->sec, insn->offset);
>  			return -1;
>  		}

Thanks, this works.  Or at least it builds, haven't booted that kernel
yet.

Cheers,
       Sven