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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Wed, 8 Jul 2020 15:19:58 +0300
Message-ID: <CAMj1kXFoXMv4MEmkpj_Ft3kA+6zm3rEL4UtRgAvEDad+rTS7cQ@mail.gmail.com>
Subject: Re: [PATCH] arm64/module-plts: Consider the special case where
 plt_max_entries is 0
To:     =?UTF-8?B?5b2t5rWpKFJpY2hhcmQp?= <richard.peng@oppo.com>
Cc:     Will Deacon <will@kernel.org>,
        "catalin.marinas@arm.com" <catalin.marinas@arm.com>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Wed, 8 Jul 2020 at 13:03, =E5=BD=AD=E6=B5=A9(Richard) <richard.peng@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 modu=
le_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@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/modul=
e-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 =3D pltsec->plt_num_entries;
> >>      int j =3D i - 1;
> >>      u64 val =3D sym->st_value + rela->r_addend;
> >> -
> >> +    if (pltsec->plt_max_entries =3D=3D 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?