From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Raphael Gault <Raphael.Gault@arm.com>
Cc: "linux-arm-kernel@lists.infradead.org"
<linux-arm-kernel@lists.infradead.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"peterz@infradead.org" <peterz@infradead.org>,
Catalin Marinas <Catalin.Marinas@arm.com>,
Will Deacon <Will.Deacon@arm.com>,
Julien Thierry <Julien.Thierry@arm.com>,
Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Subject: Re: [RFC V2 00/16] objtool: Add support for Arm64
Date: Tue, 28 May 2019 17:24:15 -0500 [thread overview]
Message-ID: <20190528222415.x63qw55ujm33dozb@treble> (raw)
In-Reply-To: <26692833-0e5b-cfe0-0ffd-c2c2f0815935@arm.com>
On Tue, May 21, 2019 at 12:50:57PM +0000, Raphael Gault wrote:
> Hi Josh,
>
> Thanks for offering your help and sorry for the late answer.
>
> My understanding is that a table of offsets is built by GCC, those
> offsets being scaled by 4 before adding them to the base label.
> I believe the offsets are stored in the .rodata section. To find the
> size of that table, it is needed to find a comparison, which can be
> optimized out apprently. In that case the end of the array can be found
> by locating labels pointing to data behind it (which is not 100% safe).
>
> On 5/16/19 3:29 PM, Josh Poimboeuf wrote:
> > On Thu, May 16, 2019 at 11:36:39AM +0100, Raphael Gault wrote:
> >> Noteworthy points:
> >> * I still haven't figured out how to detect switch-tables on arm64. I
> >> have a better understanding of them but still haven't implemented checks
> >> as it doesn't look trivial at all.
> >
> > Switch tables were tricky to get right on x86. If you share an example
> > (or even just a .o file) I can take a look. Hopefully they're somewhat
> > similar to x86 switch tables. Otherwise we may want to consider a
> > different approach (for example maybe a GCC plugin could help annotate
> > them).
> >
>
> The case which made me realize the issue is the one of
> arch/arm64/kernel/module.o:apply_relocate_add:
>
> ```
> What seems to happen in the case of module.o is:
> 334: 90000015 adrp x21, 0 <do_reloc>
> which retrieves the location of an offset in the rodata section, and a
> bit later we do some extra computation with it in order to compute the
> jump destination:
> 3e0: 78625aa0 ldrh w0, [x21, w2, uxtw #1]
> 3e4: 10000061 adr x1, 3f0 <apply_relocate_add+0xf8>
> 3e8: 8b20a820 add x0, x1, w0, sxth #2
> 3ec: d61f0000 br x0
> ```
>
> Please keep in mind that the actual offsets might vary.
>
> I'm happy to provide more details about what I have identified if you
> want me to.
I get the feeling this is going to be trickier than x86 switch tables
(which have already been tricky enough).
On x86, there's a .rela.rodata section which applies relocations to
.rodata. The presence of those relocations makes it relatively easy to
differentiate switch tables from other read-only data. For example, we
can tell that a switch table ends when either a) there's not a text
relocation or b) another switch table begins.
But with arm64 I don't see a deterministic way to do that, because the
table offsets are hard-coded in .rodata, with no relocations.
From talking with Kamalesh I got the impression that we might have a
similar issue for powerpc.
So I'm beginning to think we'll need compiler help. Like a GCC plugin
that annotates at least the following switch table metadata:
- Branch instruction address
- Switch table address
- Switch table entry size
- Switch table size
The GCC plugin could write all the above metadata into a special section
which gets discarded at link time. I can look at implementing it,
though I'll be traveling for two out of the next three weeks so it may
be a while before I can get to it.
--
Josh
next prev parent reply other threads:[~2019-05-28 22:24 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-16 10:36 [RFC V2 00/16] objtool: Add support for Arm64 Raphael Gault
2019-05-16 10:36 ` [RFC 01/16] objtool: Add abstraction for computation of symbols offsets Raphael Gault
2019-05-16 10:36 ` [RFC 02/16] objtool: orc: Refactor ORC API for other architectures to implement Raphael Gault
2019-05-16 10:36 ` [RFC 03/16] objtool: Move registers and control flow to arch-dependent code Raphael Gault
2019-05-16 10:36 ` [RFC 04/16] objtool: arm64: Add required implementation for supporting the aarch64 architecture in objtool Raphael Gault
2019-05-16 10:36 ` [RFC 05/16] objtool: arm64: Handle hypercalls as nops Raphael Gault
2019-05-16 10:36 ` [RFC 06/16] arm64: alternative: Mark .altinstr_replacement as containing executable instructions Raphael Gault
2019-05-16 10:36 ` [RFC 07/16] objtool: special: Adapt special section handling Raphael Gault
2019-05-16 10:36 ` [RFC 08/16] objtool: arm64: Adapt the stack frame checks for arm architecture Raphael Gault
2019-05-16 10:36 ` [RFC 09/16] arm64: assembler: Add macro to annotate asm function having non standard stack-frame Raphael Gault
2019-05-16 10:36 ` [RFC 10/16] arm64: sleep: Prevent stack frame warnings from objtool Raphael Gault
2019-05-16 10:36 ` [RFC 11/16] objtool: arm64: Enable stack validation for arm64 Raphael Gault
2019-05-16 10:36 ` [RFC 12/16] arm64: kvm: Annotate non-standard stack frame functions Raphael Gault
2019-05-16 10:36 ` [RFC 13/16] arm64: kernel: Add exception on kuser32 to prevent stack analysis Raphael Gault
2019-05-16 10:36 ` [RFC 14/16] arm64: crypto: Add exceptions for crypto object " Raphael Gault
2019-05-16 10:36 ` [RFC 15/16] objtool: Introduce INSN_UNKNOWN type Raphael Gault
2019-05-16 10:36 ` [RFC 16/16] arm64: kernel: Annotate non-standard stack frame functions Raphael Gault
2019-05-16 14:29 ` [RFC V2 00/16] objtool: Add support for Arm64 Josh Poimboeuf
2019-05-21 12:50 ` Raphael Gault
2019-05-22 23:11 ` Josh Poimboeuf
2019-05-28 22:24 ` Josh Poimboeuf [this message]
2019-06-13 15:55 ` Raphael Gault
2019-06-13 16:09 ` Josh Poimboeuf
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