Subject: Re: [PATCH v4 5/6] powerpc: Add show_user_instructions()

From: Murilo Opsfelder Araujo <[email-masked]>
Date: Thu, 2 Aug 2018 21:42:01 -0300

Hi, Christophe.

On Thu, Aug 02, 2018 at 07:26:20AM +0200, Christophe LEROY wrote:
>
>
> Le 01/08/2018 à 23:33, Murilo Opsfelder Araujo a écrit :
> > show_user_instructions() is a slightly modified version of
> > show_instructions() that allows userspace instruction dump.
> >
> > This will be useful within show_signal_msg() to dump userspace
> > instructions of the faulty location.
> >
> > Here is a sample of what show_user_instructions() outputs:
> >
> >    pandafault[10850]: code: 4bfffeec 4bfffee8 3c401002 38427f00 fbe1fff8 f821ffc1 7c3f0b78 3d22fffe
> >    pandafault[10850]: code: 392988d0 f93f0020 e93f0020 39400048 <99490000> 39200000 7d234b78 383f0040
> >
> > The current->comm and current->pid printed can serve as a glue that
> > links the instructions dump to its originator, allowing messages to be
> > interleaved in the logs.
> >
> > Signed-off-by: Murilo Opsfelder Araujo <[email-masked]>
> > ---
> >   arch/powerpc/include/asm/stacktrace.h | 13 +++++++++
> >   arch/powerpc/kernel/process.c         | 40 +++++++++++++++++++++++++++
> >   2 files changed, 53 insertions(+)
> >   create mode 100644 arch/powerpc/include/asm/stacktrace.h
> >
> > diff --git a/arch/powerpc/include/asm/stacktrace.h b/arch/powerpc/include/asm/stacktrace.h
> > new file mode 100644
> > index 000000000000..6149b53b3bc8
> > --- /dev/null
> > +++ b/arch/powerpc/include/asm/stacktrace.h
> > @@ -0,0 +1,13 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +/*
> > + * Stack trace functions.
> > + *
> > + * Copyright 2018, Murilo Opsfelder Araujo, IBM Corporation.
> > + */
> > +
> > +#ifndef _ASM_POWERPC_STACKTRACE_H
> > +#define _ASM_POWERPC_STACKTRACE_H
> > +
> > +void show_user_instructions(struct pt_regs *regs);
> > +
> > +#endif /* _ASM_POWERPC_STACKTRACE_H */
> > diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
> > index e9533b4d2f08..364645ac732c 100644
> > --- a/arch/powerpc/kernel/process.c
> > +++ b/arch/powerpc/kernel/process.c
> > @@ -1299,6 +1299,46 @@ static void show_instructions(struct pt_regs *regs)
> >   	pr_cont("\n");
> >   }
> > +void show_user_instructions(struct pt_regs *regs)
> > +{
> > +	int i;
> > +	const char *prefix = KERN_INFO "%s[%d]: code: ";
> > +	unsigned long pc = regs->nip - (instructions_to_print * 3 / 4 *
> > +					sizeof(int));
> > +
> > +	printk(prefix, current->comm, current->pid);
>
> Why not use pr_info() and remove KERN_INFO from *prefix ?

Because it doesn't compile:

  arch/powerpc/kernel/process.c:1317:10: error: expected ‘)’ before ‘prefix’
    pr_info(prefix, current->comm, current->pid);
            ^
  ./include/linux/printk.h:288:21: note: in definition of macro ‘pr_fmt’
   #define pr_fmt(fmt) fmt
                     ^

`pr_info(prefix, ...)` expands to `printk("\001" "6" prefix, ...)`,
which is an invalid string concatenation.

`pr_info("%s", ...)` expands to `printk("\001" "6" "%s", ...)`, which is
valid.

> > +
> > +	for (i = 0; i < instructions_to_print; i++) {
> > +		int instr;
> > +
> > +		if (!(i % 8) && (i > 0)) {
> > +			pr_cont("\n");
> > +			printk(prefix, current->comm, current->pid);
> > +		}
> > +
> > +#if !defined(CONFIG_BOOKE)
> > +		/* If executing with the IMMU off, adjust pc rather
> > +		 * than print XXXXXXXX.
> > +		 */
> > +		if (!(regs->msr & MSR_IR))
> > +			pc = (unsigned long)phys_to_virt(pc);
>
> Shouldn't this be done outside of the loop, only once ?

I don't think so.

pc gets incremented at the bottom of the loop:

  pc += sizeof(int);

Adjusting pc is necessary at each iteration.  Leaving this block inside
the loop seems correct.

Cheers
Murilo