| Message ID | 20220922215715.13345-1-will@kernel.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | vmlinux.lds.h: CFI: Reduce alignment of jump-table to function alignment | expand |
On Thu, Sep 22, 2022 at 2:57 PM Will Deacon <will@kernel.org> 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> > --- > 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 Thanks for sending the patch! Reviewed-by: Sami Tolvanen <samitolvanen@google.com> Sami
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. Heh. "That's not an image ... THIS is an image."[1] > 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. ❤️ Reviewed-by: Kees Cook <keescook@chromium.org> -Kees [1] https://www.youtube.com/watch?v=rne8pOxGuwM#t=27
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 >
On Thu, 22 Sep 2022 22:57:15 +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. > > [...] Applied to arm64 (for-next/fixes), thanks! [1/1] vmlinux.lds.h: CFI: Reduce alignment of jump-table to function alignment https://git.kernel.org/arm64/c/13b056696291 Cheers,
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
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> --- include/asm-generic/vmlinux.lds.h | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-)