Re: [PATCH v5 1/2] kprobes: textmem API

Masami Hiramatsu
2024-03-26T15:05:25Z

On Tue, 26 Mar 2024 15:18:21 +0200
"Jarkko Sakkinen" <jarkko@kernel.org> wrote:

> On Tue Mar 26, 2024 at 4:01 AM EET, Jarkko Sakkinen wrote:
> > On Tue Mar 26, 2024 at 3:31 AM EET, Jarkko Sakkinen wrote:
> > > > > +#endif /* _LINUX_EXECMEM_H */
> > > > > diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> > > > > index 9d9095e81792..87fd8c14a938 100644
> > > > > --- a/kernel/kprobes.c
> > > > > +++ b/kernel/kprobes.c
> > > > > @@ -44,6 +44,7 @@
> > > > > #include <asm/cacheflush.h>
> > > > > #include <asm/errno.h>
> > > > > #include <linux/uaccess.h>
> > > > > +#include <linux/execmem.h>
> > > > >
> > > > > #define KPROBE_HASH_BITS 6
> > > > > #define KPROBE_TABLE_SIZE (1 << KPROBE_HASH_BITS)
> > > > > @@ -113,17 +114,17 @@ enum kprobe_slot_state {
> > > > > void __weak *alloc_insn_page(void)
> > > > > {
> > > > > /*
> > > > > - * Use module_alloc() so this page is within +/- 2GB of where the
> > > > > + * Use alloc_execmem() so this page is within +/- 2GB of where the
> > > > > * kernel image and loaded module images reside. This is required
> > > > > * for most of the architectures.
> > > > > * (e.g. x86-64 needs this to handle the %rip-relative fixups.)
> > > > > */
> > > > > - return module_alloc(PAGE_SIZE);
> > > > > + return alloc_execmem(PAGE_SIZE, GFP_KERNEL);
> > > > > }
> > > > >
> > > > > static void free_insn_page(void *page)
> > > > > {
> > > > > - module_memfree(page);
> > > > > + free_execmem(page);
> > > > > }
> > > > >
> > > > > struct kprobe_insn_cache kprobe_insn_slots = {
> > > > > @@ -1580,6 +1581,7 @@ static int check_kprobe_address_safe(struct kprobe *p,
> > > > > goto out;
> > > > > }
> > > > >
> > > > > +#ifdef CONFIG_MODULES
> > > >
> > > > You don't need this block, because these APIs have dummy functions.
> > >
> > > Hmm... I'll verify this tomorrow.
> >
> > It depends on having struct module available given "(*probed_mod)->state".

Ah, indeed. We need module_state() function to avoid it.

> >
> > It is non-existent unless CONFIG_MODULES is set given how things are
> > flagged in include/linux/module.h.
>
> Hey, noticed kconfig issue.
>
> According to kconfig-language.txt:
>
> "select should be used with care. select will force a symbol to a value
> without visiting the dependencies."
>
> So the problem here lies in KPROBES config entry using select statement
> to pick ALLOC_EXECMEM. It will not take the depends on statement into
> account and thus will allow to select kprobes without any allocator in
> place.

OK, in that case "depend on" is good.

>
> So to address this I'd suggest to use depends on statement also for
> describing relation between KPROBES and ALLOC_EXECMEM. It does not make
> life worse than before for anyone because even with the current kernel
> you have to select MODULES before you can move forward with kprobes.

Yeah, since ALLOC_EXECMEM is enabled by default.

Thank you!

>
> BR, Jarkko

--
Masami Hiramatsu (Google) <mhiramat@kernel.org>