From: Mark Rutland <mark.rutland@arm.com>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: Yinan Liu <yinan@linux.alibaba.com>,
Steven Rostedt <rostedt@goodmis.org>,
"open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)"
<linuxppc-dev@lists.ozlabs.org>,
Sachin Sant <sachinp@linux.ibm.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Kees Cook <keescook@chromium.org>
Subject: Re: [powerpc] ftrace warning kernel/trace/ftrace.c:2068 with code-patching selftests
Date: Thu, 27 Jan 2022 14:54:47 +0000 [thread overview]
Message-ID: <YfKyNwYl/pkmVmDm@FVFF77S0Q05N> (raw)
In-Reply-To: <CAMj1kXHnQcHmxRrTBQmj0Z2JJ6iWvNCQqSjvPqG_oedWpikfSA@mail.gmail.com>
On Thu, Jan 27, 2022 at 02:59:31PM +0100, Ard Biesheuvel wrote:
> On Thu, 27 Jan 2022 at 14:24, Mark Rutland <mark.rutland@arm.com> wrote:
> >
> > On Thu, Jan 27, 2022 at 02:07:03PM +0100, Ard Biesheuvel wrote:
> > > I suppose that on arm64, we can work around this by passing
> > > --apply-dynamic-relocs to the linker, so that all R_AARCH64_RELATIVE
> > > targets are prepopulated with the link time value of the respective
> > > addresses. It does cause some bloat, which is why we disable that
> > > today, but we could make that dependent on ftrace being enabled.
> >
> > We'd also need to teach the build-time sort to update the relocations, unless
> > you mean to also change the boot-time reloc code to RMW with the offset?
>
> Why would that be necessary? Every RELA entry has the same effect on
> its target address, as it just adds a fixed offset.
Currently in relocate_kernel() we generate the absolute address from the
relocation alone, with the core of the relocation logic being as follows, with
x9 being the pointer to a RELA entry, and x23 being the offset relative to the
default load address:
ldp x12, x13, [x9], #24
ldr x14, [x9, #-8]
add x14, x14, x23 // relocate
str x14, [x12, x23]
... and (as per another reply), a sample RELA entry currently contains:
0xffff8000090b1ab0 // default load VA of pointer to update
0x0000000000000403 // R_AARCH64_RELATIVE
0xffff8000090b6000 // default load VA of addr to write
So either:
* That code stays as-is, and we must update the relocs to correspond to their
new sorted locations, or we'll blat the sorted values with the original
relocs as we do today.
* The code needs to change to RMW: read the existing value, add the offset
(ignoring the content of the RELA entry's addend field), and write it back.
This is what I meant when I said "change the boot-time reloc code to RMW with
the offset".
Does that make sense, or have I misunderstood?
Thanks,
Mark.
WARNING: multiple messages have this Message-ID (diff)
From: Mark Rutland <mark.rutland@arm.com>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: Kees Cook <keescook@chromium.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Steven Rostedt <rostedt@goodmis.org>,
Sachin Sant <sachinp@linux.ibm.com>,
Yinan Liu <yinan@linux.alibaba.com>,
"open list:LINUX FOR POWERPC \(32-BIT AND 64-BIT\)"
<linuxppc-dev@lists.ozlabs.org>
Subject: Re: [powerpc] ftrace warning kernel/trace/ftrace.c:2068 with code-patching selftests
Date: Thu, 27 Jan 2022 14:54:47 +0000 [thread overview]
Message-ID: <YfKyNwYl/pkmVmDm@FVFF77S0Q05N> (raw)
In-Reply-To: <CAMj1kXHnQcHmxRrTBQmj0Z2JJ6iWvNCQqSjvPqG_oedWpikfSA@mail.gmail.com>
On Thu, Jan 27, 2022 at 02:59:31PM +0100, Ard Biesheuvel wrote:
> On Thu, 27 Jan 2022 at 14:24, Mark Rutland <mark.rutland@arm.com> wrote:
> >
> > On Thu, Jan 27, 2022 at 02:07:03PM +0100, Ard Biesheuvel wrote:
> > > I suppose that on arm64, we can work around this by passing
> > > --apply-dynamic-relocs to the linker, so that all R_AARCH64_RELATIVE
> > > targets are prepopulated with the link time value of the respective
> > > addresses. It does cause some bloat, which is why we disable that
> > > today, but we could make that dependent on ftrace being enabled.
> >
> > We'd also need to teach the build-time sort to update the relocations, unless
> > you mean to also change the boot-time reloc code to RMW with the offset?
>
> Why would that be necessary? Every RELA entry has the same effect on
> its target address, as it just adds a fixed offset.
Currently in relocate_kernel() we generate the absolute address from the
relocation alone, with the core of the relocation logic being as follows, with
x9 being the pointer to a RELA entry, and x23 being the offset relative to the
default load address:
ldp x12, x13, [x9], #24
ldr x14, [x9, #-8]
add x14, x14, x23 // relocate
str x14, [x12, x23]
... and (as per another reply), a sample RELA entry currently contains:
0xffff8000090b1ab0 // default load VA of pointer to update
0x0000000000000403 // R_AARCH64_RELATIVE
0xffff8000090b6000 // default load VA of addr to write
So either:
* That code stays as-is, and we must update the relocs to correspond to their
new sorted locations, or we'll blat the sorted values with the original
relocs as we do today.
* The code needs to change to RMW: read the existing value, add the offset
(ignoring the content of the RELA entry's addend field), and write it back.
This is what I meant when I said "change the boot-time reloc code to RMW with
the offset".
Does that make sense, or have I misunderstood?
Thanks,
Mark.
next prev parent reply other threads:[~2022-01-27 14:55 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-01-24 9:19 [powerpc] ftrace warning kernel/trace/ftrace.c:2068 with code-patching selftests Sachin Sant
2022-01-24 9:19 ` Sachin Sant
2022-01-24 12:15 ` Yinan Liu
2022-01-24 16:45 ` Steven Rostedt
2022-01-25 3:20 ` Yinan Liu
2022-01-26 14:37 ` Mark Rutland
2022-01-27 11:46 ` Mark Rutland
2022-01-27 11:46 ` Mark Rutland
2022-01-27 12:03 ` Ard Biesheuvel
2022-01-27 12:03 ` Ard Biesheuvel
2022-01-27 12:20 ` Mark Rutland
2022-01-27 12:20 ` Mark Rutland
2022-01-27 12:22 ` Ard Biesheuvel
2022-01-27 12:22 ` Ard Biesheuvel
2022-01-27 12:59 ` Mark Rutland
2022-01-27 12:59 ` Mark Rutland
2022-01-27 13:07 ` Ard Biesheuvel
2022-01-27 13:07 ` Ard Biesheuvel
2022-01-27 13:24 ` Mark Rutland
2022-01-27 13:24 ` Mark Rutland
2022-01-27 13:59 ` Ard Biesheuvel
2022-01-27 13:59 ` Ard Biesheuvel
2022-01-27 14:54 ` Mark Rutland [this message]
2022-01-27 14:54 ` Mark Rutland
2022-01-27 15:01 ` Ard Biesheuvel
2022-01-27 15:01 ` Ard Biesheuvel
2022-01-27 12:04 ` Sven Schnelle
2022-01-27 12:04 ` Sven Schnelle
2022-01-27 12:27 ` Mark Rutland
2022-01-27 12:27 ` Mark Rutland
2022-01-27 12:46 ` Steven Rostedt
2022-01-27 12:46 ` Steven Rostedt
2022-01-27 13:08 ` Mark Rutland
2022-01-27 13:08 ` Mark Rutland
2022-01-27 13:16 ` Sven Schnelle
2022-01-27 13:16 ` Sven Schnelle
2022-01-27 13:33 ` Mark Rutland
2022-01-27 13:33 ` Mark Rutland
2022-01-27 13:55 ` Steven Rostedt
2022-01-27 13:55 ` Steven Rostedt
2022-01-27 14:56 ` Mark Rutland
2022-01-27 14:56 ` Mark Rutland
2022-01-27 16:41 ` Kees Cook
2022-01-27 16:41 ` Kees Cook
2022-01-25 4:00 ` Sachin Sant
2022-01-25 14:28 ` Steven Rostedt
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