On Wed, Feb 28, 2018 at 02:55:22PM +0100, Max Reitz wrote: > On 2018-02-28 14:53, Daniel P. Berrangé wrote: > > On Wed, Feb 28, 2018 at 02:45:49PM +0100, Max Reitz wrote: > >> On 2018-02-28 14:34, Daniel P. Berrangé wrote: > >>> On Wed, Feb 28, 2018 at 02:13:14PM +0100, Max Reitz wrote: > >>>> Storing the lseek() result in an int results in it overflowing when the > >>>> file is at least 2 GB big. Then, we have a 50 % chance of the result > >>>> being "negative" and thus thinking an error occurred when actually > >>>> everything went just fine. > >>>> > >>>> So we should use the correct type for storing the result: off_t. > >>>> > >>>> Reported-by: Daniel P. Berrange <berra...@redhat.com> > >>>> Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1549231 > >>>> Cc: qemu-sta...@nongnu.org > >>>> Signed-off-by: Max Reitz <mre...@redhat.com> > >>>> --- > >>>> block/file-posix.c | 5 +++-- > >>>> 1 file changed, 3 insertions(+), 2 deletions(-) > >>>> > >>>> diff --git a/block/file-posix.c b/block/file-posix.c > >>>> index f1591c3849..90c25864a0 100644 > >>>> --- a/block/file-posix.c > >>>> +++ b/block/file-posix.c > >>>> @@ -1697,6 +1697,7 @@ static int raw_regular_truncate(int fd, int64_t > >>>> offset, PreallocMode prealloc, > >>>> case PREALLOC_MODE_FULL: > >>>> { > >>>> int64_t num = 0, left = offset - current_length; > >>>> + off_t seek_result; > >>>> > >>>> /* > >>>> * Knowing the final size from the beginning could allow the > >>>> file > >>>> @@ -1711,8 +1712,8 @@ static int raw_regular_truncate(int fd, int64_t > >>>> offset, PreallocMode prealloc, > >>>> > >>>> buf = g_malloc0(65536); > >>>> > >>>> - result = lseek(fd, current_length, SEEK_SET); > >>>> - if (result < 0) { > >>>> + seek_result = lseek(fd, current_length, SEEK_SET); > >>>> + if (seek_result < 0) { > >>> > >>> off_t is an unsigned type, so this comparison to "< 0" is bogus - only the > >>> exact value (off_t)-1 indicates an error. So this needs to be > >>> > >>> if (seek_result == (off_t)-1) { > >>> ... > >>> } > >> > >> Hmmm... On my system, it appears to be a long int[1]. And > >> find_allocation() does an off_t < 0 comparison already. And > >> "man 0p sys_types.h" says "blkcnt_t and off_t shall be signed integer > >> types." > > > > Hmm, that's odd then - lseek man page explicitly said it must be cast, > > which suggested to me it could be unsigned: > > > > RETURN VALUE > > Upon successful completion, lseek() returns the resulting offset > > loca‐ > > tion as measured in bytes from the beginning of the file. On > > error, > > the value (off_t) -1 is returned and errno is set to indicate > > the > > error. > > > > CC'ing Eric for the "official" POSIX answer.... > > But it also says (under NOTES): > > The off_t data type is a signed integer data type specified by POSIX.1.
Ok, lets ignore my comments then -the "< 0" vs "!= -1" difference is harmless given this. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
