Messages in this thread | | | Date | Fri, 23 Sep 2022 13:24:20 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH] vmlinux.lds.h: CFI: Reduce alignment of jump-table to function alignment |
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On Thu, Sep 22, 2022 at 10:57:15PM +0100, Will Deacon wrote: > Due to undocumented, hysterical raisins on x86, the CFI jump-table > sections in .text are needlessly aligned to PMD_SIZE in the vmlinux > linker script. When compiling a CFI-enabled arm64 kernel with a 64KiB > page-size, a PMD maps 512MiB of virtual memory and so the .text section > increases to a whopping 940MiB and blows the final Image up to 960MiB. > Others report a link failure. > > Since the CFI jump-table requires only instruction alignment, reduce the > alignment directives to function alignment for parity with other parts > of the .text section. This reduces the size of the .text section for the > aforementioned 64KiB page size arm64 kernel to 19MiB for a much more > reasonable total Image size of 39MiB. > > Cc: Sami Tolvanen <samitolvanen@google.com> > Cc: Mark Rutland <mark.rutland@arm.com> > Cc: "Mohan Rao .vanimina" <mailtoc.mohanrao@gmail.com> > Cc: Kees Cook <keescook@chromium.org> > Cc: Nathan Chancellor <nathan@kernel.org> > Link: https://lore.kernel.org/all/CAL_GTzigiNOMYkOPX1KDnagPhJtFNqSK=1USNbS0wUL4PW6-Uw@mail.gmail.com/ > Fixes: cf68fffb66d60 ("add support for Clang CFI") > Signed-off-by: Will Deacon <will@kernel.org>
Thanks for this!
I gave this a spin atop v6.-rc3 defconfig with:
* CONFIG_ARM64_64K_PAGES=y * CONFIG_LTO_CLANG_THIN=y * CONFIG_CFI_CLANG=y
The sizes clearly speak for themselves:
| % ls -al *-v6.0-rc3-64K-CFI* | -rwxr-xr-x 1 mark mark 959693312 Sep 23 13:13 Image-v6.0-rc3-64K-CFI | -rwxr-xr-x 1 mark mark 38781440 Sep 23 13:19 Image-v6.0-rc3-64K-CFI-patched | -rwxr-xr-x 1 mark mark 1378631080 Sep 23 13:13 vmlinux-v6.0-rc3-64K-CFI | -rwxr-xr-x 1 mark mark 453015376 Sep 23 13:19 vmlinux-v6.0-rc3-64K-CFI-patched
... and before and after applying the patch, the kernel booted fine under QEMU (in TCG mode, as Cortex-A53).
I see that in scripts/module.lds.S we place the CFI bits into the .text section, and that's only aligned to PAGE_SIZE, so we don't have a similar issue for modules.
FWIW:
Reviewed-by: Mark Rutland <mark.rutland@arm.com> Tested-by: Mark Rutland <mark.rutland@arm.com>
Mark.
> --- > include/asm-generic/vmlinux.lds.h | 3 +-- > 1 file changed, 1 insertion(+), 2 deletions(-) > > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h > index 7515a465ec03..7c90b1ab3e00 100644 > --- a/include/asm-generic/vmlinux.lds.h > +++ b/include/asm-generic/vmlinux.lds.h > @@ -543,10 +543,9 @@ > */ > #ifdef CONFIG_CFI_CLANG > #define TEXT_CFI_JT \ > - . = ALIGN(PMD_SIZE); \ > + ALIGN_FUNCTION(); \ > __cfi_jt_start = .; \ > *(.text..L.cfi.jumptable .text..L.cfi.jumptable.*) \ > - . = ALIGN(PMD_SIZE); \ > __cfi_jt_end = .; > #else > #define TEXT_CFI_JT > -- > 2.37.3.998.g577e59143f-goog >
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