linux-kernel - Re: [PATCH v2] x86/boot: Correct relocation destination on old linkers

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Message-ID: <20200516231250.GA630703@rani.riverdale.lan>
Date:   Sat, 16 May 2020 19:12:50 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, "H.J. Lu" <hjl.tools@...il.com>
Subject: Re: [PATCH v2] x86/boot: Correct relocation destination on old
 linkers

On Fri, Mar 13, 2020 at 02:33:50PM -0400, Arvind Sankar wrote:
> On Fri, Feb 07, 2020 at 04:49:26PM -0500, Arvind Sankar wrote:
> > For the 32-bit kernel, as described in commit 6d92bc9d483a ("x86/build:
> > Build compressed x86 kernels as PIE"), pre-2.26 binutils generates
> > R_386_32 relocations in PIE mode.  Since the startup code does not
> > perform relocation, any reloc entry with R_386_32 will remain as 0 in
> > the executing code.
> > 
> > Commit 974f221c84b0 ("x86/boot: Move compressed kernel to the end of the
> > decompression buffer") added a new symbol _end but did not mark it
> > hidden, which doesn't give the correct offset on older linkers. This
> > causes the compressed kernel to be copied beyond the end of the
> > decompression buffer, rather than flush against it. This region of
> > memory may be reserved or already allocated for other purposes by the
> > bootloader.
> > 
> > Mark _end as hidden to fix. This changes the relocation from R_386_32 to
> > R_386_RELATIVE even on the pre-2.26 binutils.
> > 
> > For 64-bit, this is not strictly necessary, as the 64-bit kernel is only
> > built as PIE if the linker supports -z noreloc-overflow, which implies
> > binutils-2.27+, but for consistency, mark _end as hidden here too.
> > 
> 
> Gentle reminder.
> 
> https://lore.kernel.org/lkml/20200207214926.3564079-1-nivedita@alum.mit.edu/

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