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00bc526dbf
`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: 40747ffa5a ("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>
(am from https://lore.kernel.org/lkml/20200515180544.59824-1-inglorion@google.com/T/#u)
Bug: 155426344
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Change-Id: I6a5d3aeac388a0bc8254b83fa92e12376a04f922
How do I submit patches to Android Common Kernels
-
BEST: Make all of your changes to upstream Linux. If appropriate, backport to the stable releases. These patches will be merged automatically in the corresponding common kernels. If the patch is already in upstream Linux, post a backport of the patch that conforms to the patch requirements below.
- Do not send patches upstream that contain only symbol exports. To be considered for upstream Linux,
additions of
EXPORT_SYMBOL_GPL()require an in-tree modular driver that uses the symbol -- so include the new driver or changes to an existing driver in the same patchset as the export. - When sending patches upstream, the commit message must contain a clear case for why the patch is needed and beneficial to the community. Enabling out-of-tree drivers or functionality is not not a persuasive case.
- Do not send patches upstream that contain only symbol exports. To be considered for upstream Linux,
additions of
-
LESS GOOD: Develop your patches out-of-tree (from an upstream Linux point-of-view). Unless these are fixing an Android-specific bug, these are very unlikely to be accepted unless they have been coordinated with kernel-team@android.com. If you want to proceed, post a patch that conforms to the patch requirements below.
Common Kernel patch requirements
- All patches must conform to the Linux kernel coding standards and pass
script/checkpatch.pl - Patches shall not break gki_defconfig or allmodconfig builds for arm, arm64, x86, x86_64 architectures (see https://source.android.com/setup/build/building-kernels)
- If the patch is not merged from an upstream branch, the subject must be tagged with the type of patch:
UPSTREAM:,BACKPORT:,FROMGIT:,FROMLIST:, orANDROID:. - All patches must have a
Change-Id:tag (see https://gerrit-review.googlesource.com/Documentation/user-changeid.html) - If an Android bug has been assigned, there must be a
Bug:tag. - All patches must have a
Signed-off-by:tag by the author and the submitter
Additional requirements are listed below based on patch type
Requirements for backports from mainline Linux: UPSTREAM:, BACKPORT:
- If the patch is a cherry-pick from Linux mainline with no changes at all
- tag the patch subject with
UPSTREAM:. - add upstream commit information with a
(cherry picked from commit ...)line - Example:
- if the upstream commit message is
- tag the patch subject with
important patch from upstream
This is the detailed description of the important patch
Signed-off-by: Fred Jones <fred.jones@foo.org>
- then Joe Smith would upload the patch for the common kernel as
UPSTREAM: important patch from upstream
This is the detailed description of the important patch
Signed-off-by: Fred Jones <fred.jones@foo.org>
Bug: 135791357
Change-Id: I4caaaa566ea080fa148c5e768bb1a0b6f7201c01
(cherry picked from commit c31e73121f4c1ec41143423ac6ce3ce6dafdcec1)
Signed-off-by: Joe Smith <joe.smith@foo.org>
- If the patch requires any changes from the upstream version, tag the patch with
BACKPORT:instead ofUPSTREAM:.- use the same tags as
UPSTREAM: - add comments about the changes under the
(cherry picked from commit ...)line - Example:
- use the same tags as
BACKPORT: important patch from upstream
This is the detailed description of the important patch
Signed-off-by: Fred Jones <fred.jones@foo.org>
Bug: 135791357
Change-Id: I4caaaa566ea080fa148c5e768bb1a0b6f7201c01
(cherry picked from commit c31e73121f4c1ec41143423ac6ce3ce6dafdcec1)
[joe: Resolved minor conflict in drivers/foo/bar.c ]
Signed-off-by: Joe Smith <joe.smith@foo.org>
Requirements for other backports: FROMGIT:, FROMLIST:,
- If the patch has been merged into an upstream maintainer tree, but has not yet
been merged into Linux mainline
- tag the patch subject with
FROMGIT: - add info on where the patch came from as
(cherry picked from commit <sha1> <repo> <branch>). This must be a stable maintainer branch (not rebased, so don't uselinux-nextfor example). - if changes were required, use
BACKPORT: FROMGIT: - Example:
- if the commit message in the maintainer tree is
- tag the patch subject with
important patch from upstream
This is the detailed description of the important patch
Signed-off-by: Fred Jones <fred.jones@foo.org>
- then Joe Smith would upload the patch for the common kernel as
FROMGIT: important patch from upstream
This is the detailed description of the important patch
Signed-off-by: Fred Jones <fred.jones@foo.org>
Bug: 135791357
(cherry picked from commit 878a2fd9de10b03d11d2f622250285c7e63deace
https://git.kernel.org/pub/scm/linux/kernel/git/foo/bar.git test-branch)
Change-Id: I4caaaa566ea080fa148c5e768bb1a0b6f7201c01
Signed-off-by: Joe Smith <joe.smith@foo.org>
- If the patch has been submitted to LKML, but not accepted into any maintainer tree
- tag the patch subject with
FROMLIST: - add a
Link:tag with a link to the submittal on lore.kernel.org - add a
Bug:tag with the Android bug (required for patches not accepted into a maintainer tree) - if changes were required, use
BACKPORT: FROMLIST: - Example:
- tag the patch subject with
FROMLIST: important patch from upstream
This is the detailed description of the important patch
Signed-off-by: Fred Jones <fred.jones@foo.org>
Bug: 135791357
Link: https://lore.kernel.org/lkml/20190619171517.GA17557@someone.com/
Change-Id: I4caaaa566ea080fa148c5e768bb1a0b6f7201c01
Signed-off-by: Joe Smith <joe.smith@foo.org>
Requirements for Android-specific patches: ANDROID:
- If the patch is fixing a bug to Android-specific code
- tag the patch subject with
ANDROID: - add a
Fixes:tag that cites the patch with the bug - Example:
- tag the patch subject with
ANDROID: fix android-specific bug in foobar.c
This is the detailed description of the important fix
Fixes: 1234abcd2468 ("foobar: add cool feature")
Change-Id: I4caaaa566ea080fa148c5e768bb1a0b6f7201c01
Signed-off-by: Joe Smith <joe.smith@foo.org>
- If the patch is a new feature
- tag the patch subject with
ANDROID: - add a
Bug:tag with the Android bug (required for android-specific features)
- tag the patch subject with