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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

    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

    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


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      Last update: 2008-11-24 03:39    [W:3.604 / U:0.052 seconds]
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