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Re: [Qemu-block] [Qemu-devel] [PATCH 1/2] block/file-posix: Fix fully pr

From: Eric Blake
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH 1/2] block/file-posix: Fix fully preallocated truncate
Date: Wed, 28 Feb 2018 08:20:41 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0

On 02/28/2018 07:53 AM, Daniel P. Berrangé wrote:

@@ -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) {

No, off_t is ALWAYS signed, even when it is 32-bit. The cast helps code that is written for 64-bit off_t to still work when compiled with the small file model where a 32-bit value is used (but we don't have to worry about that, as we always compile for large file mode with 64-bit off_t).

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

Hmm, that's odd then - lseek man page explicitly said it must be cast,
which suggested to me it could be unsigned:

        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

CC'ing Eric for the "official" POSIX answer....

It MAY be that the man page mentions a cast because '-1' is an int but '(off_t) -1' can be larger than an int. But it may also be that you've uncovered something worth reporting as a bug to the man page project.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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