Re: [PATCH 3/9] tests/qtest/migration: Fix file migration offset check

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, May 03, 2024 at 05:36:59PM -0300, Fabiano Rosas wrote:
> Peter Xu <peterx@redhat.com> writes:
> 
> > On Fri, Apr 26, 2024 at 11:20:36AM -0300, Fabiano Rosas wrote:
> >> When doing file migration, QEMU accepts an offset that should be
> >> skipped when writing the migration stream to the file. The purpose of
> >> the offset is to allow the management layer to put its own metadata at
> >> the start of the file.
> >> 
> >> We have tests for this in migration-test, but only testing that the
> >> migration stream starts at the correct offset and not that it actually
> >> leaves the data intact. Unsurprisingly, there's been a bug in that
> >> area that the tests didn't catch.
> >> 
> >> Fix the tests to write some data to the offset region and check that
> >> it's actually there after the migration.
> >> 
> >> Fixes: 3dc35470c8 ("tests/qtest: migration-test: Add tests for file-based migration")
> >> Signed-off-by: Fabiano Rosas <farosas@suse.de>
> >> ---
> >>  tests/qtest/migration-test.c | 70 +++++++++++++++++++++++++++++++++---
> >>  1 file changed, 65 insertions(+), 5 deletions(-)
> >> 
> >> diff --git a/tests/qtest/migration-test.c b/tests/qtest/migration-test.c
> >> index 5d6d8cd634..7b177686b4 100644
> >> --- a/tests/qtest/migration-test.c
> >> +++ b/tests/qtest/migration-test.c
> >> @@ -2081,6 +2081,63 @@ static void test_precopy_file(void)
> >>      test_file_common(&args, true);
> >>  }
> >>  
> >> +#ifndef _WIN32
> >> +static void file_dirty_offset_region(void)
> >> +{
> >> +#if defined(__linux__)
> >
> > Hmm, what's the case to cover when !_WIN32 && __linux__?  Can we remove one
> > layer of ifdef?
> >
> > I'm also wondering why it can't work on win32?  I thought win32 has all
> > these stuff we used here, but I may miss something.
> >
> 
> __linux__ is because of mmap, !_WIN32 is because of the passing of
> fds. We might be able to keep !_WIN32 only, I'll check.
> 
> >> +    g_autofree char *path = g_strdup_printf("%s/%s", tmpfs, FILE_TEST_FILENAME);
> >> +    size_t size = FILE_TEST_OFFSET;
> >> +    uintptr_t *addr, *p;
> >> +    int fd;
> >> +
> >> +    fd = open(path, O_CREAT | O_RDWR, 0660);
> >> +    g_assert(fd != -1);
> >> +
> >> +    g_assert(!ftruncate(fd, size));
> >> +
> >> +    addr = mmap(NULL, size, PROT_WRITE, MAP_SHARED, fd, 0);
> >> +    g_assert(addr != MAP_FAILED);
> >> +
> >> +    /* ensure the skipped offset contains some data */
> >> +    p = addr;
> >> +    while (p < addr + FILE_TEST_OFFSET / sizeof(uintptr_t)) {
> >> +        *p = (unsigned long) FILE_TEST_FILENAME;
> >
> > This is fine, but not as clear what is assigned..  I think here we assigned
> > is the pointer pointing to the binary's RO section (rather than the chars).
> 
> Haha you're right, I was assigning the FILE_TEST_OFFSET previously and
> just switched to the FILENAME without thinking. I'll fix it up.
> 
> > Maybe using some random numbers would be more straightforward, but no
> > strong opinions.
> >
> >> +        p++;
> >> +    }
> >> +
> >> +    munmap(addr, size);
> >> +    fsync(fd);
> >> +    close(fd);
> >> +#endif
> >> +}


Use of mmap and this loop looks like overkill to me, when we can do
it in a fully portable manner with:

   g_autofree char *data = g_new0(char *, offset);
   memset(data, 0x44, offset);
   g_file_set_contents(path, data, offset, NULL);

and I checked that g_file_set_contents' impl also takes care of fsync.


With regards,
Daniel
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