Re: [PATCH v3] arm64: Allow user selection of ARM64_MODULE_PLTS
From: Will Deacon
Date: Tue Jun 18 2019 - 05:34:52 EST
On Mon, Jun 17, 2019 at 03:29:59PM -0700, Florian Fainelli wrote:
> Make ARM64_MODULE_PLTS a selectable Kconfig symbol, since some people
> might have very big modules spilling out of the dedicated module area
> into vmalloc. Help text is copied from the ARM 32-bit counterpart and
> modified to a mention of KASLR and specific ARM errata workaround(s).
> Signed-off-by: Florian Fainelli <f.fainelli@xxxxxxxxx>
> Changes in v3:
> - take out the part about "The modules will use slightly more memory, but after
> rounding up to page size, the actual memory footprint is usually the same.
> - take out the "If unusure say Y", since we would really prefer this to
> be off by default for maximum performance
> Changes in v2:
> - added Ard's paragraph about KASLR
> - added a paragraph about specific workarounds also requiring
> arch/arm64/Kconfig | 20 +++++++++++++++++++-
> 1 file changed, 19 insertions(+), 1 deletion(-)
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 697ea0510729..9206feaeff07 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -1418,8 +1418,26 @@ config ARM64_SVE
> KVM in the same kernel image.
> config ARM64_MODULE_PLTS
> - bool
> + bool "Use PLTs to allow module memory to spill over into vmalloc area"
> select HAVE_MOD_ARCH_SPECIFIC
> + help
> + Allocate PLTs when loading modules so that jumps and calls whose
> + targets are too far away for their relative offsets to be encoded
> + in the instructions themselves can be bounced via veneers in the
> + module's PLT. This allows modules to be allocated in the generic
> + vmalloc area after the dedicated module memory area has been
> + exhausted.
> + When running with address space randomization (KASLR), the module
> + region itself may be too far away for ordinary relative jumps and
> + calls, and so in that case, module PLTs are required and cannot be
> + disabled.
> + Specific errata workaround(s) might also force module PLTs to be
> + enabled (ARM64_ERRATUM_843419).
> + Disabling this is usually safe for small single-platform
> + configurations.
I think I'd still drop this last sentence, because labelling a kernel
"usually safe" without this option is a bit OTT. We'll just reject large
modules, nothing unsafe about that.
Assuming Catalin can do that when he applies, so:
Acked-by: Will Deacon <will.deacon@xxxxxxx>