linux-kernel - Re:[PATCH] arm64/module-plts: Consider the special case where plt_max

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Date:   Thu, 9 Jul 2020 06:50:18 +0000
From:   彭浩(Richard) <richard.peng@...o.com>
To:     Ard Biesheuvel <ardb@...nel.org>
CC:     Will Deacon <will@...nel.org>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re:[PATCH] arm64/module-plts: Consider the special case where
 plt_max_entries is 0

On Wed, 8 Jul 2020 at 13:03, 彭浩(Richard) <richard.peng@...o.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@...o.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.

>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.
Thanks.