Marco Elver 3ddb3fd8cd
signal, perf: Fix siginfo_t by avoiding u64 on 32-bit architectures ...
The alignment of a structure is that of its largest member. On
architectures like 32-bit Arm (but not e.g. 32-bit x86) 64-bit integers
will require 64-bit alignment and not its natural word size.
This means that there is no portable way to add 64-bit integers to
siginfo_t on 32-bit architectures without breaking the ABI, because
siginfo_t does not yet (and therefore likely never will) contain 64-bit
fields on 32-bit architectures. Adding a 64-bit integer could change the
alignment of the union after the 3 initial int si_signo, si_errno,
si_code, thus introducing 4 bytes of padding shifting the entire union,
which would break the ABI.
One alternative would be to use the __packed attribute, however, it is
non-standard C. Given siginfo_t has definitions outside the Linux kernel
in various standard libraries that can be compiled with any number of
different compilers (not just those we rely on), using non-standard
attributes on siginfo_t should be avoided to ensure portability.
In the case of the si_perf field, word size is sufficient since there is
no exact requirement on size, given the data it contains is user-defined
via perf_event_attr::sig_data. On 32-bit architectures, any excess bits
of perf_event_attr::sig_data will therefore be truncated when copying
into si_perf.
Since si_perf is intended to disambiguate events (e.g. encoding relevant
information if there are more events of the same type), 32 bits should
provide enough entropy to do so on 32-bit architectures.
For 64-bit architectures, no change is intended.
Fixes: fb6cc127e0m.szyprowski@samsung.com >
Reported-by: Jon Hunter <jonathanh@nvidia.com >
Signed-off-by: Marco Elver <elver@google.com >
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org >
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com >
Tested-by: Jon Hunter <jonathanh@nvidia.com >
Link: https://lkml.kernel.org/r/20210422191823.79012-1-elver@google.com
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