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

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Message-ID: <20200313183349.GA1544820@rani.riverdale.lan>
Date:   Fri, 13 Mar 2020 14:33:50 -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, 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/