linux-kernel - Re: [PATCH] recordmcount: avoid using ABS symbol as reference

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Message-ID: <YL492/4WrWaNDL4p@hirez.programming.kicks-ass.net>
Date:   Mon, 7 Jun 2021 17:40:11 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Mark-PK Tsai <mark-pk.tsai@...iatek.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        linux-toolchains@...r.kernel.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mediatek@...ts.infradead.org,
        Matthias Brugger <matthias.bgg@...il.com>,
        Matt Helsley <mhelsley@...are.com>,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>,
        Sami Tolvanen <samitolvanen@...gle.com>, yj.chiang@...iatek.com
Subject: Re: [PATCH] recordmcount: avoid using ABS symbol as reference

On Mon, Jun 07, 2021 at 01:44:21PM +0200, Peter Zijlstra wrote:
> One should only use st_shndx when >SHN_UDEF and <SHN_LORESERVE. When
> SHN_XINDEX, then use .symtab_shndx.
> 
> Apparently you've found a case where neither is true? In that case
> objtool seems to use shndx 0. A matching recordmcount patch would be
> something like this.
> 

Apparently I'm consistently bad at spelling SHM_UNDEF today..

> diff --git a/scripts/recordmcount.h b/scripts/recordmcount.h
> index f9b19524da11..d99cc0aed6fe 100644
> --- a/scripts/recordmcount.h
> +++ b/scripts/recordmcount.h
> @@ -194,13 +194,18 @@ static unsigned int get_symindex(Elf_Sym const *sym, Elf32_Word const *symtab,
>  	unsigned long offset;
>  	int index;
>  
> -	if (sym->st_shndx != SHN_XINDEX)
> +	if (sym->st_shndx > SHN_UDEF &&
> +	    sym->st_shndx < SHN_LORESERVE)
>  		return w2(sym->st_shndx);
>  
> -	offset = (unsigned long)sym - (unsigned long)symtab;
> -	index = offset / sizeof(*sym);
> +	if (sym->st_shndx == SHN_XINDEX) {
> +		offset = (unsigned long)sym - (unsigned long)symtab;
> +		index = offset / sizeof(*sym);
>  
> -	return w(symtab_shndx[index]);
> +		return w(symtab_shndx[index]);
> +	}
> +
> +	return 0;
>  }
>  
>  static unsigned int get_shnum(Elf_Ehdr const *ehdr, Elf_Shdr const *shdr0)