`jiffies` and `jiffies_64` are meant to alias (two different symbols that share the same address). Most architectures make the symbols alias to the same address via linker script assignment in their arch/<arch>/kernel/vmlinux.lds.S:
jiffies = jiffies_64;
which is effectively a definition of jiffies.
jiffies and jiffies_64 are both forward declared for all arch's in: include/linux/jiffies.h.
jiffies_64 is defined in kernel/time/timer.c for all arch's.
x86_64 was peculiar in that it wasn't doing the above linker script assignment, but rather was: 1. defining jiffies in arch/x86/kernel/time.c instead via linker script. 2. overriding the symbol jiffies_64 from kernel/time/timer.c in arch/x86/kernel/vmlinux.lds.s via `jiffies_64 = jiffies;`.
As Fangrui notes: ``` In LLD, symbol assignments in linker scripts override definitions in object files. GNU ld appears to have the same behavior. It would probably make sense for LLD to error "duplicate symbol" but GNU ld is unlikely to adopt for compatibility reasons. ```
So we have an ODR violation (UB), which we seem to have gotten away with thus far. Where it becomes harmful is when we:
1. Use -fno-semantic-interposition.
As Fangrui notes: ``` Clang after LLVM commit 5b22bcc2b70d ("[X86][ELF] Prefer to lower MC_GlobalAddress operands to .Lfoo$local") defaults to -fno-semantic-interposition similar semantics which help -fpic/-fPIC code avoid GOT/PLT when the referenced symbol is defined within the same translation unit. Unlike GCC -fno-semantic-interposition, Clang emits such relocations referencing local symbols for non-pic code as well. ```
This causes references to jiffies to refer to `.Ljiffies$local` when jiffies is defined in the same translation unit. Likewise, references to jiffies_64 become references to `.Ljiffies_64$local` in translation units that define jiffies_64. Because these differ from the names used in the linker script, they will not be rewritten to alias one another.
Combined with ...
2. Full LTO effectively treats all source files as one translation unit, causing these local references to be produced everywhere. When the linker processes the linker script, there are no longer any references to `jiffies_64` anywhere to replace with `jiffies`. And thus `.Ljiffies$local` and `.Ljiffies_64$local` no longer alias at all.
In the process of porting patches enabling Full LTO from arm64 to x86_64, we observe spooky bugs where the kernel appeared to boot, but init doesn't get scheduled.
Instead, we can avoid the ODR violation by matching other arch's by defining jiffies only by linker script. For -fno-semantic-interposition + Full LTO, there is no longer a global definition of jiffies for the compiler to produce a local symbol which the linker script won't ensure aliases to jiffies_64.
Link: https://github.com/ClangBuiltLinux/linux/issues/852 Fixes: 40747ffa5aa8 ("asmlinkage: Make jiffies visible") Cc: stable@vger.kernel.org Reported-by: Nathan Chancellor natechancellor@gmail.com Reported-by: Alistair Delva adelva@google.com Suggested-by: Fangrui Song maskray@google.com Debugged-by: Nick Desaulniers ndesaulniers@google.com Debugged-by: Sami Tolvanen samitolvanen@google.com Signed-off-by: Bob Haarman inglorion@google.com --- arch/x86/kernel/time.c | 4 ---- arch/x86/kernel/vmlinux.lds.S | 4 ++-- 2 files changed, 2 insertions(+), 6 deletions(-)
diff --git a/arch/x86/kernel/time.c b/arch/x86/kernel/time.c index 371a6b348e44..e42faa792c07 100644 --- a/arch/x86/kernel/time.c +++ b/arch/x86/kernel/time.c @@ -25,10 +25,6 @@ #include <asm/hpet.h> #include <asm/time.h>
-#ifdef CONFIG_X86_64 -__visible volatile unsigned long jiffies __cacheline_aligned_in_smp = INITIAL_JIFFIES; -#endif - unsigned long profile_pc(struct pt_regs *regs) { unsigned long pc = instruction_pointer(regs); diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S index 1bf7e312361f..7c35556c7827 100644 --- a/arch/x86/kernel/vmlinux.lds.S +++ b/arch/x86/kernel/vmlinux.lds.S @@ -40,13 +40,13 @@ OUTPUT_FORMAT(CONFIG_OUTPUT_FORMAT) #ifdef CONFIG_X86_32 OUTPUT_ARCH(i386) ENTRY(phys_startup_32) -jiffies = jiffies_64; #else OUTPUT_ARCH(i386:x86-64) ENTRY(phys_startup_64) -jiffies_64 = jiffies; #endif
+jiffies = jiffies_64; + #if defined(CONFIG_X86_64) /* * On 64-bit, align RODATA to 2MB so we retain large page mappings for