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Bob Haarman fadc4ae12b x86_64: Fix jiffies ODR violation 
commit d8ad6d39c3 upstream.

'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 a 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 architectures in
include/linux/jiffies.h. jiffies_64 is defined in kernel/time/timer.c.

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 the 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.

This results in an ODR violation (UB), which seems to have survived
thus far. Where it becomes harmful is when;

1. -fno-semantic-interposition is used:

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.

2. Full LTO

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,
spooky bugs have been observed where the kernel appeared to boot, but init
doesn't get scheduled.

Avoid the ODR violation by matching other architectures and define 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.

Fixes: 40747ffa5a ("asmlinkage: Make jiffies visible")
Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Reported-by: Alistair Delva <adelva@google.com>
Debugged-by: Nick Desaulniers <ndesaulniers@google.com>
Debugged-by: Sami Tolvanen <samitolvanen@google.com>
Suggested-by: Fangrui Song <maskray@google.com>
Signed-off-by: Bob Haarman <inglorion@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com> # build+boot on
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: stable@vger.kernel.org
Link: https://github.com/ClangBuiltLinux/linux/issues/852
Link: https://lkml.kernel.org/r/20200602193100.229287-1-inglorion@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2020-06-22 09:05:01 +02:00
arch x86_64: Fix jiffies ODR violation 2020-06-22 09:05:01 +02:00
block block, bfq: fix use-after-free in bfq_idle_slice_timer_body 2020-04-17 10:48:42 +02:00
certs export.h: remove VMLINUX_SYMBOL() and VMLINUX_SYMBOL_STR() 2018-08-22 23:21:44 +09:00
crypto gcc-10: avoid shadowing standard library 'free()' in crypto 2020-05-20 08:18:46 +02:00
Documentation x86/speculation: Add Ivy Bridge to affected list 2020-06-10 21:35:01 +02:00
drivers RDMA/uverbs: Make the event_queue fds return POLLERR when disassociated 2020-06-22 09:05:00 +02:00
firmware Fix built-in early-load Intel microcode alignment 2020-01-23 08:21:29 +01:00
fs btrfs: tree-checker: Check level for leaves and nodes 2020-06-22 09:05:01 +02:00
include mm: add kvfree_sensitive() for freeing sensitive data objects 2020-06-22 09:05:01 +02:00
init x86: Fix early boot crash on gcc-10, third try 2020-05-20 08:18:49 +02:00
ipc ipc/util.c: sysvipc_find_ipc() incorrectly updates position index 2020-05-20 08:18:40 +02:00
kernel sched/fair: Don't NUMA balance for kthreads 2020-06-22 09:05:00 +02:00
lib lib: Reduce user_access_begin() boundaries in strncpy_from_user() and strnlen_user() 2020-06-22 09:04:59 +02:00
LICENSES LICENSES: Remove CC-BY-SA-4.0 license text 2018-10-18 11:28:50 +02:00
mm mm: add kvfree_sensitive() for freeing sensitive data objects 2020-06-22 09:05:01 +02:00
net bridge: Avoid infinite loop when suppressing NS messages with invalid options 2020-06-22 09:04:58 +02:00
samples samples: bpf: Fix build error 2020-06-03 08:19:31 +02:00
scripts gcc-common.h: Update for GCC 10 2020-05-27 17:37:30 +02:00
security mm: add kvfree_sensitive() for freeing sensitive data objects 2020-06-22 09:05:01 +02:00
sound ALSA: hda/realtek - Add new codec supported for ALC287 2020-06-03 08:19:39 +02:00
tools perf probe: Accept the instance number of kretprobe event 2020-06-22 09:05:00 +02:00
usr initramfs: restore default compression behavior 2020-04-13 10:44:59 +02:00
virt KVM: arm64: Fix 32bit PC wrap-around 2020-05-14 07:57:20 +02:00
.clang-format clang-format: Set IndentWrappedFunctionNames false 2018-08-01 18:38:51 +02:00
.cocciconfig
.get_maintainer.ignore
.gitattributes .gitattributes: set git diff driver for C source code files 2016-10-07 18:46:30 -07:00
.gitignore Kbuild updates for v4.17 (2nd) 2018-04-15 17:21:30 -07:00
.mailmap libnvdimm-for-4.19_misc 2018-08-25 18:13:10 -07:00
COPYING COPYING: use the new text with points to the license files 2018-03-23 12:41:45 -06:00
CREDITS 9p: remove Ron Minnich from MAINTAINERS 2018-08-17 16:20:26 -07:00
Kbuild Kbuild updates for v4.15 2017-11-17 17:45:29 -08:00
Kconfig kconfig: move the "Executable file formats" menu to fs/Kconfig.binfmt 2018-08-02 08:06:55 +09:00
MAINTAINERS MAINTAINERS: Update drm/i915 bug filing URL 2020-02-28 16:38:49 +01:00
Makefile Linux 4.19.128 2020-06-10 21:35:02 +02:00
README Docs: Added a pointer to the formatted docs to README 2018-03-21 09:02:53 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
See Documentation/00-INDEX for a list of what is contained in each file.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.