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3ddb3fd8cd
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:
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|---|---|---|
| .. | ||
| accounting | ||
| arch | ||
| bootconfig | ||
| bpf | ||
| build | ||
| cgroup | ||
| debugging | ||
| edid | ||
| firewire | ||
| firmware | ||
| gpio | ||
| hv | ||
| iio | ||
| include | ||
| io_uring | ||
| kvm/kvm_stat | ||
| laptop | ||
| leds | ||
| lib | ||
| memory-model | ||
| objtool | ||
| pci | ||
| pcmcia | ||
| perf | ||
| power | ||
| scripts | ||
| spi | ||
| testing | ||
| thermal/tmon | ||
| time | ||
| tracing | ||
| usb | ||
| virtio | ||
| vm | ||
| wmi | ||
| Makefile | ||