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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:
|
||
|---|---|---|
| .. | ||
| auxvec.h | ||
| bitsperlong.h | ||
| bpf_perf_event.h | ||
| errno-base.h | ||
| errno.h | ||
| fcntl.h | ||
| hugetlb_encode.h | ||
| int-l64.h | ||
| int-ll64.h | ||
| ioctl.h | ||
| ioctls.h | ||
| ipcbuf.h | ||
| Kbuild | ||
| kvm_para.h | ||
| mman-common.h | ||
| mman.h | ||
| msgbuf.h | ||
| param.h | ||
| poll.h | ||
| posix_types.h | ||
| resource.h | ||
| sembuf.h | ||
| setup.h | ||
| shmbuf.h | ||
| siginfo.h | ||
| signal-defs.h | ||
| signal.h | ||
| socket.h | ||
| sockios.h | ||
| stat.h | ||
| statfs.h | ||
| swab.h | ||
| termbits.h | ||
| termios.h | ||
| types.h | ||
| ucontext.h | ||
| unistd.h | ||