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

[Aiqun(Maria) Yu]

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@xxxxxxxxxxx>
> >
> > 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