[PATCH] arm64/module-plts: Consider the special case where plt_max_entries is 0

[Ard Biesheuvel]

On Thu, 9 Jul 2020 at 09:50, 彭浩(Richard) <richard.peng at oppo.com> wrote:
>
> On Wed, 8 Jul 2020 at 13:03, 彭浩(Richard) <richard.peng at oppo.com> wrote:
> >>
> >>
> >> On Tue, Jul 07, 2020 at 07:46:08AM -0400, Peng Hao wrote:
> >> >> If plt_max_entries is 0, a warning is triggered.
> >> >> WARNING: CPU: 200 PID: 3000 at arch/arm64/kernel/module-plts.c:97 module_emit_plt_entry+0xa4/0x150
> >> >
> >> > Which kernel are you seeing this with? There is a PLT-related change in
> >> > for-next/core, and I'd like to rule if out if possible.
> >> >
> >> 5.6.0-rc3+
> >> >> Signed-off-by: Peng Hao <richard.peng at oppo.com>
> >> >> ---
> >> >>  arch/arm64/kernel/module-plts.c | 3 ++-
> >> >>  1 file changed, 2 insertions(+), 1 deletion(-)
> >> >>
> >> >> diff --git a/arch/arm64/kernel/module-plts.c b/arch/arm64/kernel/module-plts.c
> >> >> index 65b08a74aec6..1868c9ac13f2 100644
> >> >> --- a/arch/arm64/kernel/module-plts.c
> >> >> +++ b/arch/arm64/kernel/module-plts.c
> >> >> @@ -79,7 +79,8 @@ u64 module_emit_plt_entry(struct module *mod, Elf64_Shdr *sechdrs,
> >> >>      int i = pltsec->plt_num_entries;
> >> >>      int j = i - 1;
> >> >>      u64 val = sym->st_value + rela->r_addend;
> >> >> -
> >> >> +    if (pltsec->plt_max_entries == 0)
> >> >> +            return 0;
> >> >
> >> >Hmm, but if there aren't any PLTs then how do we end up here?
> >> >
> >> We also returned 0 when warning was triggered.
> >
> >That doesn't really answer the question.
> >
> >Apparently, you are hitting a R_AARCH64_JUMP26 or R_AARCH64_CALL26
> >relocation that operates on a b or bl instruction that is more than
> >128 megabytes away from its target.
> >
> My understanding is that a module that calls functions that are not part of the module will use PLT.
> Plt_max_entries =0 May occur if a module does not depend on other module functions.
>

A PLT slot is allocated for each b or bl instruction that refers to a
symbol that lives in a different section, either of the same module
(e.g., bl in .init calling into .text), of another module, or of the
core kernel.

I don't see how you end up with plt_max_entries in this case, though.
Are you sure you have CONFIG_RANDOMIZE_BASE enabled?

> >In module_frob_arch_sections(), we count all such relocations that
> >point to other sections, and allocate a PLT slot for each (and update
> >plt_max_entries) accordingly. So this means that the relocation in
> >question was disregarded, and this could happen for only two reasons:
> >- the branch instruction and its target are both in the same section,
> >in which case this section is *really* large,
> >- CONFIG_RANDOMIZE_BASE is disabled, but you are still ending up in a
> >situation where the modules are really far away from the core kernel
> >or from other modules.
> >
> >Do you have a lot of [large] modules loaded when this happens?
> I don’t think I have [large] modules.  I'll trace which module caused this warning.

Yes please.