native_save_fl() is marked static inline, but by using it as a function pointer in arch/x86/kernel/paravirt.c, it MUST be outlined.
paravirt's use of native_save_fl() also requires that no GPRs other than %rax are clobbered. Compilers have different heuristics which they use to emit stack guard code, the emittance of which can break paravirt's callee saved assumption by clobbering %rcx. Marking a function definition extern inline means that if this version cannot be inlined, then the out-of-line version will be preferred. By having the out-of-line version be implemented in assembly, it cannot be instrumented with a stack protector, which might violate custom calling conventions that code like paravirt rely on. The semantics of extern inline has changed since gnu89. This means that folks using GCC versions >= 5.1 may see symbol redefinition errors at link time for subdirs that override KBUILD_CFLAGS (making the C standard used implicit) regardless of this patch. This has been cleaned up earlier in the patch set, but is left as a note in the commit message for future travelers. Reports: https://lkml.org/lkml/2018/5/7/534 https://github.com/ClangBuiltLinux/linux/issues/16 Discussion: https://bugs.llvm.org/show_bug.cgi?id=37512 https://lkml.org/lkml/2018/5/24/1371 Thanks to the many folks that participated in the discussion. Acked-by: Juergen Gross <jgr...@suse.com> Debugged-by: Alistair Strachan <astrac...@google.com> Debugged-by: Matthias Kaehlcke <m...@chromium.org> Reported-by: Sedat Dilek <sedat.di...@gmail.com> Signed-off-by: Nick Desaulniers <ndesaulni...@google.com> Suggested-by: Arnd Bergmann <a...@arndb.de> Suggested-by: H. Peter Anvin <h...@zytor.com> Suggested-by: Tom Stellar <tstel...@redhat.com> Tested-by: Sedat Dilek <sedat.di...@gmail.com> --- Backport for 4.4. 4.4 is missing commit 784d5699eddc "x86: move exports to actual definitions" which doesn't apply cleanly, and not really worth backporting IMO. It's simpler to change this patch from upstream: + #include <asm-generic/export.h> rather than + #include <asm/export.h> arch/x86/include/asm/irqflags.h | 2 +- arch/x86/kernel/Makefile | 1 + arch/x86/kernel/irqflags.S | 26 ++++++++++++++++++++++++++ 3 files changed, 28 insertions(+), 1 deletion(-) create mode 100644 arch/x86/kernel/irqflags.S diff --git a/arch/x86/include/asm/irqflags.h b/arch/x86/include/asm/irqflags.h index b77f5edb03b0..0056bc945cd1 100644 --- a/arch/x86/include/asm/irqflags.h +++ b/arch/x86/include/asm/irqflags.h @@ -8,7 +8,7 @@ * Interrupt control: */ -static inline unsigned long native_save_fl(void) +extern inline unsigned long native_save_fl(void) { unsigned long flags; diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile index b1b78ffe01d0..7947cee61f61 100644 --- a/arch/x86/kernel/Makefile +++ b/arch/x86/kernel/Makefile @@ -41,6 +41,7 @@ obj-y += alternative.o i8253.o pci-nommu.o hw_breakpoint.o obj-y += tsc.o tsc_msr.o io_delay.o rtc.o obj-y += pci-iommu_table.o obj-y += resource.o +obj-y += irqflags.o obj-y += process.o obj-y += fpu/ diff --git a/arch/x86/kernel/irqflags.S b/arch/x86/kernel/irqflags.S new file mode 100644 index 000000000000..3817eb748eb4 --- /dev/null +++ b/arch/x86/kernel/irqflags.S @@ -0,0 +1,26 @@ +/* SPDX-License-Identifier: GPL-2.0 */ + +#include <asm/asm.h> +#include <asm-generic/export.h> +#include <linux/linkage.h> + +/* + * unsigned long native_save_fl(void) + */ +ENTRY(native_save_fl) + pushf + pop %_ASM_AX + ret +ENDPROC(native_save_fl) +EXPORT_SYMBOL(native_save_fl) + +/* + * void native_restore_fl(unsigned long flags) + * %eax/%rdi: flags + */ +ENTRY(native_restore_fl) + push %_ASM_ARG1 + popf + ret +ENDPROC(native_restore_fl) +EXPORT_SYMBOL(native_restore_fl) -- 2.18.0.233.g985f88cf7e-goog
