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Message-ID: <20161028175749.GB28451@obsidianresearch.com>
Date: Fri, 28 Oct 2016 11:57:49 -0600
From: Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To: Russell King <linux@...linux.org.uk>,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [ARM] Fix stack alignment when processing backtraces
On Tue, Oct 18, 2016 at 11:05:10AM -0600, Jason Gunthorpe wrote:
> The dumpstm helper within c_backtrace pushed 5 dwords onto the stack
> causing the stack to become unaligned and then calls printk. This
> causes memory corruption in the kernel which assumes AAPCS calling
> convention.
>
> Since this bit of asm doesn't use the standard prologue just add
> another register to restore alignment.
>
> Fixes: 7ab3f8d595a1b ("[ARM] Add ability to dump exception stacks to kernel backtraces")
> Signed-off-by: Jason Gunthorpe <jgunthorpe@...idianresearch.com>
> arch/arm/lib/backtrace.S | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> In my case the kernel was hitting a WARN_ON during boot and then
> reliably failed to start the compiled-in initramfs.
>
> I'm inferring that the stack misalignment caused some kind of memory
> corruption which wiped out the unpacked initramfs.
>
> Saw with gcc 5.4.0 on a kirkwood armv5te
Since there are no comments, I will send this to RMK's patch system..
Thanks,
Jason
> diff --git a/arch/arm/lib/backtrace.S b/arch/arm/lib/backtrace.S
> index fab5a50503ae..25e1cce19991 100644
> +++ b/arch/arm/lib/backtrace.S
> @@ -116,7 +116,8 @@ ENDPROC(c_backtrace)
> #define reg r5
> #define stack r6
>
> -.Ldumpstm: stmfd sp!, {instr, reg, stack, r7, lr}
> + /* Must maintain 8 byte stack alignment */
> +.Ldumpstm: stmfd sp!, {r3, instr, reg, stack, r7, lr}
> mov stack, r0
> mov instr, r1
> mov reg, #10
> @@ -140,7 +141,7 @@ ENDPROC(c_backtrace)
> teq r7, #0
> adrne r0, .Lcr
> blne printk
> - ldmfd sp!, {instr, reg, stack, r7, pc}
> + ldmfd sp!, {r3, instr, reg, stack, r7, pc}
>
> .Lfp: .asciz " r%d:%08x%s"
> .Lcr: .asciz "\n"
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