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I realize that we fairly recently raised it to 4.8, but the fact is, 4.9
is a much better minimum version to target.
We have a number of workarounds for actual bugs in pre-4.9 gcc versions
(including things like internal compiler errors on ARM), but we also
have some syntactic workarounds for lacking features.
In particular, raising the minimum to 4.9 means that we can now just
assume _Generic() exists, which is likely the much better replacement
for a lot of very convoluted built-time magic with conditionals on
sizeof and/or __builtin_choose_expr() with same_type() etc.
Using _Generic also means that you will need to have a very recent
version of 'sparse', but thats easy to build yourself, and much less of
a hassle than some old gcc version can be.
The latest (in a long string) of reasons for minimum compiler version
upgrades was commit 5435f73d5c4a ("efi/x86: Fix build with gcc 4").
Ard points out that RHEL 7 uses gcc-4.8, but the people who stay back on
old RHEL versions persumably also don't build their own kernels anyway.
And maybe they should cross-built or just have a little side affair with
a newer compiler?
Acked-by: Ard Biesheuvel <[email protected]>
Acked-by: Peter Zijlstra <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
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GENMASK() and GENMASK_ULL() are supposed to be called with the high bit as
the first argument and the low bit as the second argument. Mixing them
will return a mask with zero bits set.
Recent commits show getting this wrong is not uncommon, see e.g. commit
aa4c0c9091b0 ("net: stmmac: Fix misuses of GENMASK macro") and commit
9bdd7bb3a844 ("clocksource/drivers/npcm: Fix misuse of GENMASK macro").
To prevent such mistakes from appearing again, add compile time sanity
checking to the arguments of GENMASK() and GENMASK_ULL(). If both
arguments are known at compile time, and the low bit is higher than the
high bit, break the build to detect the mistake immediately.
Since GENMASK() is used in declarations, BUILD_BUG_ON_ZERO() must be used
instead of BUILD_BUG_ON().
__builtin_constant_p does not evaluate is argument, it only checks if it
is a constant or not at compile time, and __builtin_choose_expr does not
evaluate the expression that is not chosen. Therefore, GENMASK(x++, 0)
does only evaluate x++ once.
Commit 95b980d62d52 ("linux/bits.h: make BIT(), GENMASK(), and friends
available in assembly") made the macros in linux/bits.h available in
assembly. Since BUILD_BUG_OR_ZERO() is not asm compatible, disable the
checks if the file is included in an asm file.
Due to bugs in GCC versions before 4.9 [0], disable the check if building
with a too old GCC compiler.
[0]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19449
Signed-off-by: Rikard Falkeborn <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Reviewed-by: Masahiro Yamada <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Geert Uytterhoeven <[email protected]>
Cc: Haren Myneni <[email protected]>
Cc: Joe Perches <[email protected]>
Cc: Johannes Berg <[email protected]>
Cc: lkml <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>
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The vDSO library should only include the necessary headers required for
a userspace library (UAPI and a minimal set of kernel headers). To make
this possible it is necessary to isolate from the kernel headers the
common parts that are strictly necessary to build the library.
Split bits.h into linux and common headers to make the latter suitable
for inclusion in the vDSO library.
Signed-off-by: Vincenzo Frascino <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
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BIT(), GENMASK(), etc. are useful to define register bits of hardware.
However, low-level code is often written in assembly, where they are
not available due to the hard-coded 1UL, 0UL.
In fact, in-kernel headers such as arch/arm64/include/asm/sysreg.h
use _BITUL() instead of BIT() so that the register bit macros are
available in assembly.
Using macros in include/uapi/linux/const.h have two reasons:
[1] For use in uapi headers
We should use underscore-prefixed variants for user-space.
[2] For use in assembly code
Since _BITUL() uses UL(1) instead of 1UL, it can be used as an
alternative of BIT().
For [2], it is pretty easy to change BIT() etc. for use in assembly.
This allows to replace _BUTUL() in kernel-space headers with BIT().
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Masahiro Yamada <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Christian Borntraeger <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: Vasily Gorbik <[email protected]>
Cc: Vineet Gupta <[email protected]>
Cc: Will Deacon <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
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new <linux/bits.h> file
In preparation for implementing the asm-generic atomic bitops in terms
of atomic_long_*(), we need to prevent <asm/atomic.h> implementations from
pulling in <linux/bitops.h>. A common reason for this include is for the
BITS_PER_BYTE definition, so move this and some other BIT() and masking
macros into a new header file, <linux/bits.h>.
Signed-off-by: Will Deacon <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: https://lore.kernel.org/lkml/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>
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