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

[Ard Biesheuvel]

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.

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?