Re: [PATCH] arm64: module: PLT allowed even !RANDOM_BASE

["Aiqun(Maria) Yu" <quic_aiquny@quicinc.com>]

On 10/23/2023 5:02 PM, Mark Rutland wrote:
> On Mon, Oct 23, 2023 at 10:08:33AM +0200, Arnd Bergmann wrote:
>> On Mon, Oct 23, 2023, at 09:57, Maria Yu wrote:
>>> Module PLT feature can be enabled even when RANDOM_BASE is disabled.
>>> Break BLT entry counts of relocation types will make module plt entry
>>> allocation fail and finally exec format error for even correct and plt
>>> allocation available modules.
> 
> Has an actual problem been seen in practice, or was this found by looking at
> the code?
I've encounter an actual problem when disalbe CONFIG_RADOM_BASE and the 
kernel module have the exec format error issue.
> 
>>>
>>> Signed-off-by: Maria Yu <quic_aiquny@quicinc.com>
>>
>> Adding Ard Biesheuvel to Cc, as he added the check in commit
>> a257e02579e42 ("arm64/kernel: don't ban ADRP to work around
>> Cortex-A53 erratum #843419")
Thx for adding Ard. Will keep him in next patchset as well.
> 
> I think that the actual mistake is in commit:
> 
>    3e35d303ab7d22c4 ("arm64: module: rework module VA range selection")
> 
> Prior to that commit, when CONFIG_RANDOMIZE_BASE=n all modules and code had to
> be within 128M of each other, and so there were no PLTs necessary for B/BL.
> After that commit we can have a 2G module range regardless of
> CONFIG_RANDOMIZE_BASE, and PLTs may be necessary for B/BL.
> 
> We should have removed the check for !CONFIG_RANDOMIZE_BASE as part of that.

Agree with you.
> 
>>>   arch/arm64/kernel/module-plts.c | 3 ---
>>>   1 file changed, 3 deletions(-)
>>>
>>> diff --git a/arch/arm64/kernel/module-plts.c
>>> b/arch/arm64/kernel/module-plts.c
>>> index bd69a4e7cd60..21a67d52d7a0 100644
>>> --- a/arch/arm64/kernel/module-plts.c
>>> +++ b/arch/arm64/kernel/module-plts.c
>>> @@ -167,9 +167,6 @@ static unsigned int count_plts(Elf64_Sym *syms,
>>> Elf64_Rela *rela, int num,
>>>   		switch (ELF64_R_TYPE(rela[i].r_info)) {
>>>   		case R_AARCH64_JUMP26:
>>>   		case R_AARCH64_CALL26:
>>> -			if (!IS_ENABLED(CONFIG_RANDOMIZE_BASE))
>>> -				break;
>>> -
>>>   			/*
>>>   			 * We only have to consider branch targets that resolve
>>>   			 * to symbols that are defined in a different section.
>>
>> I see there are two such checks (in partition_branch_plt_relas()
>> and in count_plts()), can you explain in more detail how you
>> concluded that one of them is correct but the other one is not?
> 
> I believe that the one in partition_branch_plt_relas() needs to go too; that's
> just a minor optimization for the case where there shouldn't be any PLTs for
> B/BL, and it no longer holds after the module VA range rework.
> 
> That was introduced in commit:
> 
>    d4e0340919fb9190 ("arm64/module: Optimize module load time by optimizing PLT counting")
The functionality is the same from my try with plt allocated kernel 
modules. While the PLT entry can be dramatically reduced from ~50000 to 
~500 after fix in partition_branch_plt_relas.

I will send out the second patchset with fix in 
partition_branch_plt_relas (remove check of CONFIG_RANDOMIZE_BASE) 
tomorrow if no more other comments today.
> 
> Thanks,
> Mark.

-- 
Thx and BRs,
Aiqun(Maria) Yu