From: Sedat Dilek <sedat.dilek@gmail.com>
To: Bob Haarman <inglorion@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Andrew Morton <akpm@linux-foundation.org>,
Josh Poimboeuf <jpoimboe@redhat.com>,
stable@vger.kernel.org,
Nathan Chancellor <natechancellor@gmail.com>,
Alistair Delva <adelva@google.com>,
Fangrui Song <maskray@google.com>,
Nick Desaulniers <ndesaulniers@google.com>,
Sami Tolvanen <samitolvanen@google.com>,
Andi Kleen <ak@linux.intel.com>,
x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
afzal mohammed <afzal.mohd.ma@gmail.com>,
Kyung Min Park <kyung.min.park@intel.com>,
Peter Zijlstra <peterz@infradead.org>,
Kees Cook <keescook@chromium.org>,
Heiko Carstens <heiko.carstens@de.ibm.com>,
Baoquan He <bhe@redhat.com>,
Thomas Lendacky <Thomas.Lendacky@amd.com>,
"H.J. Lu" <hjl.tools@gmail.com>,
Ross Zwisler <zwisler@chromium.org>,
Arvind Sankar <nivedita@alum.mit.edu>,
Dmitry Safonov <0x7f454c46@gmail.com>,
linux-kernel@vger.kernel.org,
Clang-Built-Linux ML <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH v2] x86_64: fix jiffies ODR violation
Date: Thu, 4 Jun 2020 09:30:13 +0200 [thread overview]
Message-ID: <CA+icZUWe3ezev3rQJO4rJ5trSPoSfUX+eOBqPXbJhRe-EZQ6-w@mail.gmail.com> (raw)
In-Reply-To: <20200602193100.229287-1-inglorion@google.com>
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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next prev parent reply other threads:[~2020-06-04 7:30 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-15 18:05 [PATCH] x86_64: fix jiffies ODR violation Bob Haarman
2020-05-19 3:17 ` Andi Kleen
2020-06-01 23:47 ` Bob Haarman
2020-06-02 13:27 ` Josh Poimboeuf
2020-06-02 19:30 ` [PATCH v2] " Bob Haarman
2020-06-04 7:30 ` Sedat Dilek [this message]
2020-06-09 8:53 ` [tip: x86/urgent] x86_64: Fix " tip-bot2 for Bob Haarman
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