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* Re:[PATCH] arm64/module-plts: Consider the special case where plt_max_entries is 0
@ 2020-07-09  6:50 彭浩(Richard)
  2020-07-09  6:55   ` Ard Biesheuvel
  0 siblings, 1 reply; 4+ messages in thread
From: 彭浩(Richard) @ 2020-07-09  6:50 UTC (permalink / raw)
  To: Ard Biesheuvel
  Cc: Will Deacon, catalin.marinas, linux-arm-kernel, linux-kernel

On Wed, 8 Jul 2020 at 13:03, 彭浩(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 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@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.

>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.

^ permalink raw reply	[flat|nested] 4+ messages in thread
* Re:[PATCH] arm64/module-plts: Consider the special case where plt_max_entries is 0
@ 2020-07-09  7:18 彭浩(Richard)
  0 siblings, 0 replies; 4+ messages in thread
From: 彭浩(Richard) @ 2020-07-09  7:18 UTC (permalink / raw)
  To: Ard Biesheuvel
  Cc: Will Deacon, catalin.marinas, linux-arm-kernel, linux-kernel

On Thu, 9 Jul 2020 at 09:50, 彭浩(Richard) <richard.peng@oppo.com> wrote:
>> >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.
if a module does not depend on other module functions, PLT entries in the module is equal to 0.
If this is the case I don't think I need to do anything, just return 0.What do you think should be 
done about this situation? Any Suggestions?
Thanks.
>Are you sure you have CONFIG_RANDOMIZE_BASE enabled?
>
CONFIG_RANDOMIZE_BASE = y or n has this warning (two servers, kernel version is different).

>> >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.
I can't print debug until someone else is not using the server.

^ permalink raw reply	[flat|nested] 4+ messages in thread
end of thread, other threads:[~2020-07-09  7:18 UTC | newest]

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2020-07-09  6:50 Re:[PATCH] arm64/module-plts: Consider the special case where plt_max_entries is 0 彭浩(Richard)
2020-07-09  6:55 ` [PATCH] " Ard Biesheuvel
2020-07-09  6:55   ` Ard Biesheuvel
2020-07-09  7:18 彭浩(Richard)

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