From: Arnd Bergmann > Sent: 12 April 2021 12:26 > > On Mon, Apr 12, 2021 at 12:54 PM David Laight <David.Laight@xxxxxxxxxx> wrote: > > From: David Laight > Sent: 12 April 2021 10:37 > > ... > > > I'm guessing that compat_pid_t is 16 bits? > > > So the native 32bit version has an unnamed 2 byte structure pad. > > > The 'packed' removes this pad from the compat structure. > > > > > > AFAICT (apart from mips) the __ARCH_COMPAT_FLOCK_PAD is just > > > adding an explicit pad for the implicit pad the compiler > > > would generate because compat_pid_t is 16 bits. > > > > I've just looked at the header. > > compat_pid_t is 32 bits. > > So Linux must have gained 32bit pids at some earlier time. > > (Historically Unix pids were 16 bit - even on 32bit systems.) > > > > Which makes the explicit pad in 'sparc' rather 'interesting'. > > I saw it was there since the sparc kernel support got merged in > linux-1.3, possibly copied from an older sunos version. Which had a 16bit pid when I used it. So this is a bug in the sparc merge! The explicit 'short' pad could be removed from the 64bit variant because there are always 4 bytes of pad after l_pid. But it does extend the application structure on 32bit sparc so must remain in the uapi header. It doesn't need to be in the 'compat' definition. > > oh - compat_loff_t is only used in a couple of other places. > > neither care in any way about the alignment. > > (Provided get_user() doesn't fault on a 8n+4 aligned address.) > > Ah right, I also see that after this series it's only used in to other > places: compat_resume_swap_area, which could also lose the > __packed annotation, That structure just defines 0 and 8, the structure size doesn't matter and the offsets are 'passed to' get_user() so byte accesses aren't performed. > and in the declaration of > compat_sys_sendfile64, where it makes no difference. Which should probably use get_user() rather than copy_from_user(). Although some architectures may need fallback code for misaligned get_user() ? Or is there a general 'cop out' that structures passed to the kernel are required to be correctly aligned. They should be aligned unless the kernel is 'playing games' like reading 'struct pollfd' as a 64bit item. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
