linux-kernel - Re: [PATCH v3 04/12] module: Add printk format to add module build ID to stacktraces

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Date:   Thu, 08 Apr 2021 12:57:08 -0700
From:   Stephen Boyd <swboyd@...omium.org>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Jessica Yu <jeyu@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Jiri Olsa <jolsa@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Evan Green <evgreen@...omium.org>,
        Hsin-Yi Wang <hsinyi@...omium.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-doc@...r.kernel.org, Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v3 04/12] module: Add printk format to add module build ID to stacktraces

Quoting Andy Shevchenko (2021-04-08 07:05:54)
> On Thu, Apr 08, 2021 at 03:44:57PM +0200, Jessica Yu wrote:
> > +++ Stephen Boyd [30/03/21 20:05 -0700]:
> 
> ...
> 
> > > +static void init_build_id(struct module *mod, const struct load_info *info)
> > > +{
> > > +   const Elf_Shdr *sechdr;
> > > +   unsigned int i;
> > > +
> > > +   for (i = 0; i < info->hdr->e_shnum; i++) {
> > > +           sechdr = &info->sechdrs[i];
> > > +           if (!sect_empty(sechdr) && sechdr->sh_type == SHT_NOTE &&
> > > +               !build_id_parse_buf((void *)sechdr->sh_addr, mod->build_id,
> > > +                                   sechdr->sh_size))
> > > +                   break;
> > > +   }
> > > +}
> > 
> > Why not just look for the .note.gnu.build-id section instead of trying
> > to parse each note section? Doesn't it always contain the build id? At
> > least the ld man page seems to suggest this section name should be
> > consistent.

That's basically what this code is doing. We're looking through all the
section headers and finding the ones that are notes and then
build_id_parse_buf() is checking to see if that note is a GNU type note
(name == "GNU") and is of the type NT_GNU_BUILD_ID (type == 3). We don't
need to check for a section name of ".note.gnu.build-id", we can use the
existing code in build_id_parse_buf() that looks for the name and type.

> 
> Interesting idea (in positive way!), I'm wondering what Clang does in such
> case.
> 

Clang also inserts a GNU build ID and it works with these patches.