Hi Ingo,
Here is a small pile of cleanups and fixes for nospec.h after inspection from Linus, Rasmus, and Christian. Full changelogs below:
These have received a build success notification from 0day across 126 configs.
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Dan Williams (2): nospec: Kill array_index_nospec_mask_check() nospec: Include asm/barrier.h dependency
Rasmus Villemoes (1): nospec: Allow index argument to have const-qualified type
include/linux/nospec.h | 26 +++----------------------- 1 file changed, 3 insertions(+), 23 deletions(-)
-- nospec: Kill array_index_nospec_mask_check()
There are multiple problems with the dynamic sanity checking in array_index_nospec_mask_check():
* It causes unnecessary overhead in the 32-bit case since integer sized @index values will no longer cause the check to be compiled away like in the 64-bit case.
* In the 32-bit case it may trigger with user controllable input when the expectation is that should only trigger during development of new kernel enabling.
* The macro reuses the input parameter in multiple locations which is broken if someone passes an expression like 'index++' to array_index_nospec().
nospec: Allow index argument to have const-qualified type
The last expression in a statement expression need not be a bare variable, quoting gcc docs
The last thing in the compound statement should be an expression followed by a semicolon; the value of this subexpression serves as the value of the entire construct.
and we already use that in e.g. the min/max macros which end with a ternary expression.
This way, we can allow index to have const-qualified type, which will in some cases avoid the need for introducing a local copy of index of non-const qualified type. That, in turn, can prevent readers not familiar with the internals of array_index_nospec from wondering about the seemingly redundant extra variable, and I think that's worthwhile considering how confusing the whole _nospec business is.
The expression _i&_mask has type unsigned long (since that is the type of _mask, and the BUILD_BUG_ONs guarantee that _i will get promoted to that), so in order not to change the type of the whole expression, add a cast back to typeof(_i).
nospec: Include asm/barrier.h dependency
The nospec.h header expects the per-architecture header file asm/barrier.h to optionally define array_index_mask_nospec(). Include that dependency to prevent inadvertent fallback to the default array_index_mask_nospec() implementation. The default implementation may not provide a full mitigation on architectures that perform data value speculation.