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    SubjectRe: [patch 1/7] Immediate Values - Architecture Independent Code
    Date
    On Tuesday 18 September 2007 21:47, Mathieu Desnoyers wrote:
    > * Denys Vlasenko (vda.linux@googlemail.com) wrote:
    > > On Tuesday 18 September 2007 18:59, Mathieu Desnoyers wrote:
    > > > * Denys Vlasenko (vda.linux@googlemail.com) wrote:
    > > > > On Monday 17 September 2007 19:42, Mathieu Desnoyers wrote:
    > > > > > Index: linux-2.6-lttng/include/asm-generic/vmlinux.lds.h
    > > > > > ===================================================================
    > > > > > --- linux-2.6-lttng.orig/include/asm-generic/vmlinux.lds.h 2007-09-17 13:25:06.000000000 -0400
    > > > > > +++ linux-2.6-lttng/include/asm-generic/vmlinux.lds.h 2007-09-17 13:35:50.000000000 -0400
    > > > > > @@ -122,6 +122,13 @@
    > > > > > VMLINUX_SYMBOL(__stop___kcrctab_gpl_future) = .; \
    > > > > > } \
    > > > > > \
    > > > > > + /* Immediate values: pointers */ \
    > > > > > + __immediate : AT(ADDR(__immediate) - LOAD_OFFSET) { \
    > > > > > + VMLINUX_SYMBOL(__start___immediate) = .; \
    > > > > > + *(__immediate) \
    > > > > > + VMLINUX_SYMBOL(__stop___immediate) = .; \
    > > > > > + } \
    > > > > > + \
    > > > >
    > > > > Why do you need an output section for that? IOW: will this work too?
    > > > >
    > > > > .data : ... {
    > > > > ...
    > > > >
    > > > > VMLINUX_SYMBOL(__start___immediate) = .; \
    > > > > *(__immediate) \
    > > > > VMLINUX_SYMBOL(__stop___immediate) = .; \
    > > > > ...
    > > > > }
    > > > >
    > > >
    > > > This last one could cause alignment problems. We either have to use the
    > > > proper ALIGN() before the section, or let AT(ADDR(__immediate) -
    > > > LOAD_OFFSET) take care of it. I prefer the latter.
    > >
    > > This adds yet another output section in vmlinux, and there is
    > > no tools which need that. We already have 30+ sections there while we need ~20.
    > >
    > > I am trying to fix the mess. Please don't add to it.
    > >
    > > Re alignment: (1) do you really realy REALLY need it? Last I checked,
    > > i386 was handling unaligned accesses just fine; and
    > > (2) this works:
    > >
    > > . = ALIGN(4)
    > > VMLINUX_SYMBOL(__start___immediate) = .; \
    > > *(__immediate) \
    > > VMLINUX_SYMBOL(__stop___immediate) = .; \
    > >
    > >
    >
    > Alignment: I need the __start___immediate and __stop___immediate values
    > to be at the same alignment as the *(__immediate) content, or else we
    > end up thinking that padding is data.
    >
    > . = ALIGN(4) works fine as long as the structure within the section is
    > not bigger or equal to 32 bytes: gcc has the habit to align 32 bytes
    > structure on 32 bytes multiples. The safest way I found to do it is to
    > declare the section as I do: it will cause no breakage if anybody append
    > data to the structure.

    If your structure will be padded by gcc, then this:

    +#define immediate_read(name) \
    + ({ \
    + __typeof__(name##__immediate) value; \
    + switch (sizeof(value)) { \
    + case 1: \
    + asm ( ".section __immediate, \"a\", @progbits;\n\t" \
    + ".long %1, (0f)+1, 1;\n\t" \
    + ".previous;\n\t" \
    + "0:\n\t" \
    + "mov %2,%0;\n\t" \
    + : "=r" (value) \
    + : "m" (name##__immediate), \
    + "i" (0)); \
    + break; \

    will produce wrongly-sized "struct __immediate" (truncated one),
    since gcc has no idea that you are building struct __immediate there,
    and here:

    +void immediate_update_range(const struct __immediate *begin,
    + const struct __immediate *end)
    +{
    + const struct __immediate *iter;
    + int ret;
    +
    + for (iter = begin; iter < end; iter++) {
    + mutex_lock(&immediate_mutex);
    + kernel_text_lock();
    + ret = arch_immediate_update(iter);
    + kernel_text_unlock();
    + if (ret)
    + printk(KERN_WARNING "Invalid immediate value. "
    + "Variable at %p, "
    + "instruction at %p, size %lu\n",
    + (void*)iter->immediate,
    + (void*)iter->var, iter->size);
    + mutex_unlock(&immediate_mutex);
    + }
    +}

    iter++ will go off rails.
    --
    vda
    -
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