Re: [PATCH 2/8] ARM: aarch64: Fix get_runtime_offset after relocation

From: Sascha Hauer <s.hauer@pengutronix.de>
To: Andrey Smirnov <andrew.smirnov@gmail.com>
Cc: Barebox List <barebox@lists.infradead.org>
Subject: Re: [PATCH 2/8] ARM: aarch64: Fix get_runtime_offset after relocation
Date: Fri, 23 Aug 2019 10:11:05 +0200	[thread overview]
Message-ID: <20190823081105.znj2f4r7jjx3npfn@pengutronix.de> (raw)
In-Reply-To: <CAHQ1cqHb2EtQ_CCFW7PYOyR-yZ86Fx0605gSwsx_fxUojYffKw@mail.gmail.com>

On Thu, Aug 22, 2019 at 01:14:23PM -0700, Andrey Smirnov wrote:
> On Thu, Aug 22, 2019 at 5:52 AM Sascha Hauer <s.hauer@pengutronix.de> wrote:
> >
> > get_runtime_offset shall return the offset between the address we are
> > running at and the address we are linked at. This value obviously
> > changes when we relocate the binary. cf3b09737b tried to avoid using
> > R_AARCH64_RELATIVE relocations, but in fact this is exactly what the
> > function needs to work. Consider barebox starting at 0x10000000
> > when we are linked at 0x0 then get_runtime_offset() should return
> > 0x10000000 before relocate_to_current_adr(), but afterwards it should
> > return 0x0.
> >
> > This patch brings back the previously removed "a" flag. Since gcc5
> > doesn't put the values of R_AARCH64_RELATIVE fixup'd relocations
> > into the binary but zeroes instead, we help ourselves by basing
> > get_runtime_offset on an address which actually is zero. With
> > CONFIG_RELOCATABLE=y the binary is always linked to 0x0, so _text
> > is initially zero.
> >
> 
> This paragraph might be worth putting in the comment as well to
> document some cleverness that is still required to deal with GCC5.

Makes sence. Added this:

/*
 * With older gcc versions (gcc5) function pointers will not be filled
 * into the binary during compile time and instead rely on relocation
 * during runtime. In the binary we'll always have 0x0 here. We deliberately
 * use _text here since that is 0x0 and is correct without relocation.
 */

Sascha

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