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SubjectRe: [PATCH 00/14] Fix wrong %pF and %pS printk format specifier usages
On (09/08/17 20:28), Helge Deller wrote:
[..]
> I don't like this kind of trying to figure out at runtime at all.
> It's too much guessing in here IMHO.

well, may be we can avoid any guessing by checking that the
pointer belongs to .opd section.

for kernel we can add 2 new unsigned longs - __opd_start __opd_end -- and
tweak the corresponding arch/${FOO}/kernel/vmlinux.lds.S file to set those
two, the same way text, unwinding, etc. are set.

.... wait a second.

arch/ia64/kernel/vmlinux.lds.S already handles .opd section

.opd : AT(ADDR(.opd) - LOAD_OFFSET) {
*(.opd)
}

it just doesn't save start/end addresses. so all we need to to
is

.opd : AT(ADDR(.opd) - LOAD_OFFSET) {
+ __opd_start = .;
*(.opd)
+ __opd_end = .;
}

and tweak symbol dereference

static inline void *dereference_function_descriptor(void *ptr)
{
struct fdesc *desc = ptr;
void *p;

+ if (prt < (void *)__start_opd || (void *)__end_opd < ptr)
+ return ptr;
+
if (!probe_kernel_address(&desc->ip, p))
ptr = p;
return ptr;



now, the modules.

module_frob_arch_sections() has the following lines

else if (strcmp(".opd", secstrings + s->sh_name) == 0)
mod->arch.opd = s;

so, once, again, we keep the .opd section info in memory. and we
also have the size of that section

mod->arch.opd->sh_size = fdescs * sizeof(struct fdesc);

so it seems that we've got what we need. need to provide arch callback
(same way as we do with dereference_function_descriptor() to properly
dereference modules' symbols).


so I think we almost have what we need to make ps/pS smart enough
on ppc64/ia64/parisc.

powerpc and parisc handle kernel .opd section as well:

arch/powerpc/kernel/vmlinux.lds.S: .opd
arch/parisc/kernel/vmlinux.lds.S: .opd


need to check more.


> What about this idea:
> For %pF we always have pointers to functions, e.g.:
> printk("Going to call: %pF\n", gettimeofday);
> printk("Going to call: %pF\n", p->func);
>
> and for %pS most (if not all) usages use some kind of casting
> from "unsigned long" to "void *", e.g.:
> printk("%s: called from %pS\n", __func__, (void *)_RET_IP_);
> printk("%s: called from %pS\n", __func__, (void *)__builtin_return_address(0));
> printk("Faulted at %pS\n", (void *)regs->ip);
>
> So, what if we for the %pS case simply take the type as it is
> (unsigned long) and introduce a new printk-format, e.g. "%luS" ?
> The %pS examples above then become:
> printk("%s: called from %luS\n", __func__, _RET_IP_);
> printk("%s: called from %luS\n", __func__, __builtin_return_address(0));
> printk("Faulted at %luS\n", regs->ip);
>
> That way we don't need type-casting, gain compile-time type
> checks from the compiler, and we could add a checkpatch (or occinelle)
> check which checks for the combination of %pF/%pS and "void*" keyword
> and suggest to use %luS.
>
> Opinions?

hm. sounds interesting. but I'm afraid people won't be so happy
to learn a new printk format specifier.

-ss

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