linux-kernel - Re: [powerpc] ftrace warning kernel/trace/ftrace.c:2068 with code-patching selftests

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Date:   Thu, 27 Jan 2022 12:27:04 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Sven Schnelle <svens@...ux.ibm.com>
Cc:     Yinan Liu <yinan@...ux.alibaba.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linuxppc-dev@...ts.ozlabs.org, Sachin Sant <sachinp@...ux.ibm.com>,
        linux-kernel@...r.kernel.org, ardb@...nel.org,
        keescook@...omium.org
Subject: Re: [powerpc] ftrace warning kernel/trace/ftrace.c:2068 with
 code-patching selftests

On Thu, Jan 27, 2022 at 01:04:41PM +0100, Sven Schnelle wrote:
> Mark Rutland <mark.rutland@....com> writes:
> 
> >> Isn't x86 relocatable in some configurations (e.g. for KASLR)?
> >> 
> >> I can't see how the sort works for those cases, because the mcount_loc entries
> >> are absolute, and either:
> >> 
> >> * The sorted entries will get overwritten by the unsorted relocation entries,
> >>   and won't be sorted.
> >> 
> >> * The sorted entries won't get overwritten, but then the absolute address will
> >>   be wrong since they hadn't been relocated.
> >> 
> >> How does that work?
> 
> From what i've seen when looking into this ftrace sort problem x86 has a
> a relocation tool, which is run before final linking: arch/x86/tools/relocs.c
> This tools converts all the required relocations to three types:
> 
> - 32 bit relocations
> - 64 bit relocations
> - inverse 32 bit relocations
> 
> These are added to the end of the image.
> 
> The decompressor then iterates over that array, and just adds/subtracts
> the KASLR offset - see arch/x86/boot/compressed/misc.c, handle_relocations()
> 
> So IMHO x86 never uses 'real' relocations during boot, and just
> adds/subtracts. That's why the order stays the same, and the compile
> time sort works.

Ah, so those non-ELF relocations for the mcount_loc table just mean "apply the
KASLR offset here", which is equivalent for all entries.

That makes sense, thanks!

Mark.