Date: Mon, 24 Nov 2008 15:59:05 -0500 (EST)
From: Steven Rostedt <rostedt@goodmis.org>
To: Paul Mackerras <paulus@samba.org>
cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@elte.hu>,
       Andrew Morton <akpm@linux-foundation.org>,
       Benjamin Herrenschmidt <benh@kernel.crashing.org>,
       Thomas Gleixner <tglx@linutronix.de>, linuxppc-dev@ozlabs.org,
       Milton Miller <miltonm@bga.com>, Steven Rostedt <srostedt@redhat.com>
Subject: Re: [PATCH 4/5] powerpc/ppc64: ftrace, handle module trampolines
 for dyn ftrace
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References: <20081120190948.057007623@goodmis.org> <20081120191149.759009300@goodmis.org> <18730.4326.745599.500581@drongo.ozlabs.ibm.com>
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Paul and Ingo,

Would it be best for me to just refold these changes into the original 
patch series, and update the git repo? Or should I apply these changes 
on top of this series?

Thanks,

-- Steve


On Mon, 24 Nov 2008, Paul Mackerras wrote:

> Steven Rostedt writes:
> 
> > +#ifdef CONFIG_PPC64
> > +static int
> > +__ftrace_make_nop(struct module *mod,
> > +		  struct dyn_ftrace *rec, unsigned long addr)
> > +{
> > +	unsigned char replaced[MCOUNT_INSN_SIZE * 2];
> > +	unsigned int *op = (unsigned *)&replaced;
> 
> This makes me a little nervous, since it looks to me to be breaking
> aliasing rules.  I know we use -fno-strict-aliasing, but still it
> would be better to avoid doing these casts if possible - and we should
> be able to avoid most of them by using unsigned int for instructions
> consistently, instead of a mix of unsigned int and unsigned char.
> 
> > +	DEBUGP("ip:%lx jumps to %lx r2: %lx", ip, tramp, mod->arch.toc);
> > +
> > +	/* Find where the trampoline jumps to */
> > +	if (probe_kernel_read(jmp, (void *)tramp, 8)) {
> > +		printk(KERN_ERR "Failed to read %lx\n", tramp);
> > +		return -EFAULT;
> > +	}
> > +
> > +	DEBUGP(" %08x %08x",
> > +	       (unsigned)(*ptr >> 32),
> > +	       (unsigned)*ptr);
> > +
> > +	offset = (unsigned)jmp[2] << 24 |
> > +		(unsigned)jmp[3] << 16 |
> > +		(unsigned)jmp[6] << 8 |
> > +		(unsigned)jmp[7];
> 
> We don't seem to be checking that these instructions look like the
> start of a trampoline created by module_64.c, which makes me a little
> nervous.
> 
> If the kernel text goes over 32MB, the linker will insert trampolines
> automatically.  Those trampolines either look like a direct branch to
> the target, or else they look like this:
> 
> 	addis	r12,r2,xxxx
> 	ld	r11,yyyy(r12)
> 	mtctr	r11
> 	bctr
> 
> where xxxx/yyyy gives the offset from the kernel TOC to the procedure
> descriptor for the target.
> 
> Now, a kernel with > 32MB of text probably won't work for other
> reasons at the moment (like the linker putting trampolines before the
> interrupt vectors), so in a sense it doesn't matter.  It also doesn't
> matter since we only get here for calls in modules (something that
> could stand to be mentioned in a comment at the top of the function).
> Nevertheless, I think it would be worthwhile to check that the first
> two instructions look like the addis and addi that we are expecting.
> 
> > +	if (probe_kernel_write((void *)ip, replaced, MCOUNT_INSN_SIZE))
> > +		return -EPERM;
> > +
> > +	return 0;
> > +}
> 
> We don't seem to do anything to ensure I-cache consistency.  I think
> we probably need a flush_icache_range call here.  Similarly in
> __ftrace_make_call.
> 
> Paul.
> 
> 
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