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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