linux-kernel - Re: build failure after commit eb0481bbc4ce ("objtool: Fix reloc

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Date:   Wed, 28 Jun 2023 21:32:31 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     Michal Kubecek <mkubecek@...e.cz>, linux-kernel@...r.kernel.org
Subject: Re: build failure after commit eb0481bbc4ce ("objtool: Fix
 reloc_hash size")

On Wed, Jun 28, 2023 at 11:10:19AM -0700, Josh Poimboeuf wrote:
> On Wed, Jun 28, 2023 at 07:26:59PM +0200, Michal Kubecek wrote:
> > I digged some more and my guess is that the problem is that
> > elf_open_read() does
> > 
> > 	memset(elf, 0, offsetof(struct elf, sections));
> > 
> > but commit eb0481bbc4ce ("objtool: Fix reloc_hash size") added
> > num_relocs after sections so that it is not zeroed (0xbabababababababa
> > is probably some kind of poison).
> 
> Argh, that memset() is subtle as heck.  Peter, why?!?

Well, at the time struct elf had a bunch of fairly large hash tables
embedded inside it. memset() on those was a significant performance fail
-- esp. since we called objtool on every .o file.

 struct elf {
   Elf *elf;
   GElf_Ehdr ehdr;
   int fd;
   char *name;
   struct list_head sections;
   DECLARE_HASHTABLE(symbol_hash, 20);
   DECLARE_HASHTABLE(symbol_name_hash, 20);
   DECLARE_HASHTABLE(section_hash, 16);
   DECLARE_HASHTABLE(section_name_hash, 16);
   DECLARE_HASHTABLE(rela_hash, 20);
 };

Those embedded hash-tables have gone away, but apparently someone forgot
about this thing :/

Yes, this can go, struct elf is no longer like that.