Re: [PULL v2 02/16] block/file-posix: introduce helper functions for sysfs attributes

[Sam Li]

Matthew Rosato <mjrosato@linux.ibm.com> 于2023年6月3日周六 02:41写道：
>
> On 6/2/23 2:18 PM, Sam Li wrote:
> > Matthew Rosato <mjrosato@linux.ibm.com> 于2023年6月1日周四 02:21写道：
> >>
> >> On 5/15/23 12:04 PM, Stefan Hajnoczi wrote:
> >>> From: Sam Li <faithilikerun@gmail.com>
> >>>
> >>> Use get_sysfs_str_val() to get the string value of device
> >>> zoned model. Then get_sysfs_zoned_model() can convert it to
> >>> BlockZoneModel type of QEMU.
> >>>
> >>> Use get_sysfs_long_val() to get the long value of zoned device
> >>> information.
> >>
> >> Hi Stefan, Sam,
> >>
> >> I am having an issue on s390x using virtio-blk-{pci,ccw} backed by an NVMe 
> >> partition, and I've bisected the root cause to this commit.
> >>
> >> I noticed that tests which use the partition e.g. /dev/nvme0n1p1 as a 
> >> backing device would fail, but those that use the namespace e.g. 
> >> /dev/nvme0n1 would still succeed.  The root issue appears to be that the 
> >> block device associated with the partition does not have a "max_segments" 
> >> attribute, and prior to this patch hdev_get_max_segment() would return 
> >> -ENOENT in this case.  After this patch, however, QEMU is instead 
> >> crashing.  It looks like g_file_get_contents is returning 0 with a len == 
> >> 0 if the specified sysfs path does not exist.  The following diff on top 
> >> seems to resolve the issue for me:
> >>
> >>
> >> diff --git a/block/file-posix.c b/block/file-posix.c
> >> index 0ab158efba2..eeb0247c74e 100644
> >> --- a/block/file-posix.c
> >> +++ b/block/file-posix.c
> >> @@ -1243,7 +1243,7 @@ static int get_sysfs_str_val(struct stat *st, const 
> >> char *attribute,
> >>                                  major(st->st_rdev), minor(st->st_rdev),
> >>                                  attribute);
> >>      ret = g_file_get_contents(sysfspath, val, &len, NULL);
> >> -    if (ret == -1) {
> >> +    if (ret == -1 || len == 0) {
> >>          return -ENOENT;
> >>      }
> >>
> >
> > Hi Matthew,
> >
> > Thanks for the information. After some checking, I think the bug here
> > is that g_file_get_contens returns g_boolean value and the error case
> > will return 0 instead of -1 in my previous code. Can the following
> > line fix your issue on the s390x device?
> >
> > + if (ret == FALSE) {
> >
> > https://docs.gtk.org/glib/func.file_get_contents.html
>
> Hi Sam,
>
> Ah, good point, I didn't notice file_get_contents was meant to be a bool 
> return and wondered why I was getting a return of 0 in the failing case, 
> hence the check for len == 0.
>
> Anyway, yes, I verified that checking for ret == FALSE fixes the issue.  
> FWIW, along the same line I also checked that this works:
>
>     if (!g_file_get_contents(sysfspath, val, &len, NULL)) {
>         return -ENOENT;
>     }
>
> which I personally think looks cleaner and matches the other uses of 
> g_file_get_contents in QEMU.  Could also get rid of ret and just return 0 at 
> the bottom of the function.

Indeed. I will fix this. Thanks!

Sam