Re: [PATCH v2] x86_64: fix jiffies ODR violation

[Sedat Dilek]

On Tue, Jun 2, 2020 at 9:31 PM 'Bob Haarman' via Clang Built Linux
<clang-built-linux@xxxxxxxxxxxxxxxx> wrote:
>
> '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@xxxxxxxxxxxxxxx
> Reported-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
> Reported-by: Alistair Delva <adelva@xxxxxxxxxx>
> Suggested-by: Fangrui Song <maskray@xxxxxxxxxx>
> Debugged-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> Debugged-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
> Signed-off-by: Bob Haarman <inglorion@xxxxxxxxxx>
> Reviewed-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Reviewed-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> ---
> v2:
> * Changed commit message as requested by Josh Poimboeuf
>   (no code change)
>

Hi,

I have tested v2 with my local patch-series together.

Feel free to add my...

Tested-by: Sedat Dilek <sedat.dilek@xxxxxxxxx> # build+boot on
Debian/testing AMD64 with selfmade llvm-toolchain v10.0.1-rc1+

Thanks.

Regards,
- Sedat -

> ---
>  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
> --
> 2.27.0.rc2.251.g90737beb825-goog
>
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