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SubjectRe: [PATCH 4/5] powerpc/ppc64: ftrace, handle module trampolines for dyn ftrace
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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