[v2] x86_64: fix jiffies ODR violation
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Message ID 20200602193100.229287-1-inglorion@google.com
State In Next
Commit d8ad6d39c35d2b44b3d48b787df7f3359381dcbf
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  • [v2] x86_64: fix jiffies ODR violation
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Commit Message

Bob Haarman June 2, 2020, 7:30 p.m. UTC
'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>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
v2:
* Changed commit message as requested by Josh Poimboeuf
  (no code change)

---
 arch/x86/kernel/time.c        | 4 ----
 arch/x86/kernel/vmlinux.lds.S | 4 ++--
 2 files changed, 2 insertions(+), 6 deletions(-)

Comments

Sedat Dilek June 4, 2020, 7:30 a.m. UTC | #1
On Tue, Jun 2, 2020 at 9:31 PM 'Bob Haarman' via Clang Built Linux
<clang-built-linux@googlegroups.com> 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@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>
> Reviewed-by: Andi Kleen <ak@linux.intel.com>
> Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
> ---
> 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@gmail.com> # 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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Patch
diff mbox series

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