Re: [PATCH 6/7] modules: Add CONFIG_ARCH_WANTS_MODULES_DATA_IN_VMALLOC

[Luis Chamberlain <mcgrof@kernel.org>]

On Wed, Jan 26, 2022 at 06:38:30AM +0000, Christophe Leroy wrote:
> 
> 
> Le 25/01/2022 à 22:10, Luis Chamberlain a écrit :
> > On Mon, Jan 24, 2022 at 09:22:34AM +0000, Christophe Leroy wrote:
> >> This can also be useful on other powerpc/32 in order to maximize the
> >> chance of code being close enough to kernel core to avoid branch
> >> trampolines.
> > 
> > Curious about all this branch trampoline talk. Do you have data to show
> > negative impact with things as-is?
> 
> See 
> https://github.com/linuxppc/linux/commit/2ec13df167040cd153c25c4d96d0ffc573ac4c40
> 
> Or 
> https://github.com/linuxppc/linux/commit/7d485f647c1f4a6976264c90447fb0dbf07b111d


This was useful and fun to read, thanks.

> > Also, was powerpc/32 broken then without this? The commit log seems to
> > suggest so, but I don't think that's the case. How was this issue noticed?
> 
> 
> Your question is related to the trampoline topic or the exec/noexec 
> flagging ?
> 
> Regarding trampoline, everything is working OK. That's just cherry on 
> the cake, when putting data away you can have more code closer to the 
> kernel. But that would not have been a reason in itself for this series.
> 
> Regarding the exec/noexec discussion, it's a real issue. powerpc/32 
> doesn't honor page exec flag, so when you select STRICT_MODULES_RWX and 
> flag module data as no-exec, it remains executable. That's because 
> powerpc/32 MMU doesn't have a per page exec flag but only a per 
> 256Mbytes segment exec flag.
> 
> Typical PPC32 layount:
> 0xf0000000-0xffffffff : VMALLOC AREA ==> NO EXEC
> 0xc0000000-0xefffffff : Linear kernel memory mapping
> 0xb0000000-0xbfffffff : MODULES AREA ==> EXEC
> 0x00000000-0xafffffff : User space ==> EXEC
> 
> So STRICT_MODULES_RWX is broken on some powerpc/32

You know, this is the sort of information that I think would be
very useful for the commit log. Can you ammend?

> > 
> > Are there other future CPU families being planned where this is all true for
> > as well? Are they goin to be 32-bit as well?
> 
> Future I don't know.
> 
> Regarding the trampoline stuff, I see at least the following existing 
> architectures with a similar constraint:
> 
> ARM:
> 
> https://elixir.bootlin.com/linux/v5.16/source/arch/arm/include/asm/memory.h#L55
> 
> ARM even has a config item to allow trampolines or not. I might add the 
> same to powerpc to reduce number of pages used by modules.
> 
> https://elixir.bootlin.com/linux/v5.16/source/arch/arm/Kconfig#L1514
> 
> NDS32 has the constraint
> 
> https://elixir.bootlin.com/linux/v5.16/source/arch/nds32/include/asm/memory.h#L41
> 
> NIOS2 has the constraint, allthough they handled it in a different way:
> 
> https://elixir.bootlin.com/linux/v5.16/source/arch/nios2/kernel/module.c#L30
> 
> 
> 
> Even ARM64 benefits from modules closer to kernel:
> 
> https://elixir.bootlin.com/linux/v5.16/source/arch/arm64/Kconfig#L1848
> 
> 
> Another future opportunity with the ability to allocate module parts 
> separately is the possibility to then use huge vmalloc mappings.
> 
> Today huge vmalloc mappings cannot be used for modules, see recent 
> discussion at 
> https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20211227145903.187152-4-wangkefeng.wang@huawei.com/

Alrighty, this is sufficient information, thanks!

  Luis