On Sat, Jan 18, 2020 at 8:57 AM Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
>
> The final build stage of the x86 kernel captures some symbol
> addresses from the decompressor binary and copies them into zoffset.h.
> It uses sed with a regular expression that matches the address, symbol
> type and symbol name, and mangles the captured addresses and the names
> of symbols of interest into #define directives that are added to
> zoffset.h
>
> The symbol type is indicated by a single letter, which we match
> strictly: only letters in the set 'ABCDGRSTVW' are matched, even
> though the actual symbol type is relevant and therefore ignored.
>
> Commit bc7c9d620 ("efi/libstub/x86: Force 'hidden' visibility for
> extern declarations") made a change to the way external symbol
> references are classified, resulting in 'startup_32' now being
> emitted as a hidden symbol. This prevents the use of GOT entries to
> refer to this symbol via its absolute address, which recent toolchains
> (including Clang based ones) already avoid by default, making this
> change a no-op in the majority of cases.
>
> However, as it turns out, the LLVM linker classifies such hidden
> symbols as symbols with static linkage in fully linked ELF binaries,
> causing tools such as NM to output a lowercase 't' rather than an upper
> case 'T' for the type of such symbols. Since our sed expression only
> matches upper case letters for the symbol type, the line describing
> startup_32 is disregarded, resulting in a build error like the following
>
> arch/x86/boot/header.S:568:18: error: symbol 'ZO_startup_32' can not be
> undefined in a subtraction expression
> init_size: .long (0x00000000008fd000 - ZO_startup_32 +
> (((0x0000000001f6361c + ((0x0000000001f6361c >> 8) + 65536)
> - 0x00000000008c32e5) + 4095) & ~4095)) # kernel initialization size
>
> Given that we are only interested in the value of the symbol, let's match
> any character in the set 'a-zA-Z' instead.
>
> Tested-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
> Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Thanks for the patch! This fixes a build breakage for us.
Tested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Link: https://github.com/ClangBuiltLinux/linux/issues/842
> Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> ---
> arch/x86/boot/Makefile | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/boot/Makefile b/arch/x86/boot/Makefile
> index 95410d6ee2ff..748b6d28a91d 100644
> --- a/arch/x86/boot/Makefile
> +++ b/arch/x86/boot/Makefile
> @@ -88,7 +88,7 @@ $(obj)/vmlinux.bin: $(obj)/compressed/vmlinux FORCE
>
> SETUP_OBJS = $(addprefix $(obj)/,$(setup-y))
>
> -sed-zoffset := -e 's/^\([0-9a-fA-F]*\) [ABCDGRSTVW] \(startup_32\|startup_64\|efi32_stub_entry\|efi64_stub_entry\|efi_pe_entry\|input_data\|kernel_info\|_end\|_ehead\|_text\|z_.*\)$$/\#define ZO_\2 0x\1/p'
> +sed-zoffset := -e 's/^\([0-9a-fA-F]*\) [a-zA-Z] \(startup_32\|startup_64\|efi32_stub_entry\|efi64_stub_entry\|efi_pe_entry\|input_data\|kernel_info\|_end\|_ehead\|_text\|z_.*\)$$/\#define ZO_\2 0x\1/p'
>
> quiet_cmd_zoffset = ZOFFSET $@
> cmd_zoffset = $(NM) $< | sed -n $(sed-zoffset) > $@
> --
> 2.17.1
>
--
Thanks,
~Nick Desaulniers
