On Sat, Mar 11, 2017 at 4:16 AM, Mike Frysinger <vapier@xxxxxxxxxx> wrote: > Architectures that split 64-bit values across registers pairs usually do > so according to their C ABI calling convention (which means endianness). > Add some notes to that effect, and change the readahead example to show > a little endian example (since that is way more common than big endian). > > Also start a new list of syscalls that this issue does not apply to. > Maybe add preadv/prwritev to that list? The whole story which promted me to dig deep into syscall(2) page started with pread/preadv story (thankfully pwrite acts as pread and pwritev acts as preadv, no surprises there). They have the following prototypes: ssize_t pread(int fd, void *buf, size_t count, off_t offset); ssize_t preadv(int fd, const struct iovec *iov, int iovcnt, off_t offset); As you could see they are delightfully similar (interpreteation of parameters are different, but types are almost the same) but my attempts to handle them invariably broken one or other. Eventually I've found article which explained what goes on: https://lwn.net/Articles/311630/ - apparently preadv was made different from pread to make life easier (not sure who's life that made easier, though)! > Signed-off-by: Mike Frysinger <vapier@xxxxxxxxxx> > --- > man2/syscall.2 | 12 ++++++++++-- > 1 file changed, 10 insertions(+), 2 deletions(-) > > diff --git a/man2/syscall.2 b/man2/syscall.2 > index fcb0a7e21248..80f83fd1aaac 100644 > --- a/man2/syscall.2 > +++ b/man2/syscall.2 > @@ -102,13 +102,14 @@ Thus, using > instead of the wrapper provided by glibc, > the > .BR readahead () > -system call would be invoked as follows on the ARM architecture with the EABI: > +system call would be invoked as follows on the ARM architecture with the EABI > +in little endian mode: > > .in +4n > .nf > syscall(SYS_readahead, fd, 0, > - (unsigned int) (offset >> 32), > (unsigned int) (offset & 0xFFFFFFFF), > + (unsigned int) (offset >> 32), > count); > .fi > .in > @@ -124,6 +125,8 @@ register pair. > That means inserting a dummy value into > .I r1 > (the second argument of 0). > +Care also must be taken so that the split follows endian conventions > +(according to the C ABI for the platform). > > Similar issues can occur on MIPS with the O32 ABI, > on PowerPC with the 32-bit ABI, and on Xtensa. > @@ -140,6 +143,11 @@ The affected system calls are > .BR sync_file_range (2), > and > .BR truncate64 (2). > + > +This does not affect syscalls that manually split and assemble 64-bit values > +such as > +.BR _llseek (2). > +Welcome to the wonderful world of historical baggage. > .SS Architecture calling conventions > Every architecture has its own way of invoking and passing arguments to the > kernel. > -- > 2.12.0 > -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
