Re: [Qemu-devel] [PATCH v3] bugfix: passing reference instead of value

[Stefano Stabellini]

On Sat, 2 Jan 2016, Michael S. Tsirkin wrote:
> On Sat, Jan 02, 2016 at 04:02:20PM +0800, Cao jin wrote:
> > Fix the bug introduced by 595a4f07: function host_pci_config_read() should 
> > be
> > pass-by-reference, not value.
> > 
> > Signed-off-by: Cao jin <address@hidden>
> > ---
> > v3 changelog:
> > 1. Remove cpu_to_le32() since the code only runs on X86.
> 
> It really should be le32_to_cpu and a separate patch,
> but I think it's preferable to have it there
> since people tend to copy code around.
> 
> But in any case, before merging any patches in this function I'd like to
> hear a response from someone explaining why is this function necessary
> at all, since it provably never did anything useful.

If Tiejun's email address bounces, then we are unlikely to get a reply.

I think that the pass-by-value bug was introduced in one of the
rebase/resend versions, as the series is very old and originally looked
very different. I would take the patch as is to fix the obvious bug.

This is how the original code looks like:

http://xenbits.xen.org/gitweb/?p=qemu-xen-traditional.git;a=blob_plain;f=hw/pt-graphics.c;hb=HEAD

See the function named igd_pci_read.





> > 
> >  hw/pci-host/piix.c | 8 +++++---
> >  1 file changed, 5 insertions(+), 3 deletions(-)
> > 
> > diff --git a/hw/pci-host/piix.c b/hw/pci-host/piix.c
> > index 715208b..924f0fa 100644
> > --- a/hw/pci-host/piix.c
> > +++ b/hw/pci-host/piix.c
> > @@ -761,7 +761,7 @@ static const IGDHostInfo igd_host_bridge_infos[] = {
> >      {0xa8, 4},  /* SNB: base of GTT stolen memory */
> >  };
> >  
> > -static int host_pci_config_read(int pos, int len, uint32_t val)
> > +static int host_pci_config_read(int pos, int len, uint32_t *val)
> >  {
> >      char path[PATH_MAX];
> >      int config_fd;
> > @@ -784,12 +784,14 @@ static int host_pci_config_read(int pos, int len, 
> > uint32_t val)
> >          ret = -errno;
> >          goto out;
> >      }
> > +
> >      do {
> > -        rc = read(config_fd, (uint8_t *)&val, len);
> > +        rc = read(config_fd, (uint8_t *)val, len);
> >      } while (rc < 0 && (errno == EINTR || errno == EAGAIN));
> >      if (rc != len) {
> >          ret = -errno;
> >      }
> > +
> >  out:
> >      close(config_fd);
> >      return ret;
> > @@ -805,7 +807,7 @@ static int igd_pt_i440fx_initfn(struct PCIDevice 
> > *pci_dev)
> >      for (i = 0; i < num; i++) {
> >          pos = igd_host_bridge_infos[i].offset;
> >          len = igd_host_bridge_infos[i].len;
> > -        rc = host_pci_config_read(pos, len, val);
> > +        rc = host_pci_config_read(pos, len, &val);
> >          if (rc) {
> >              return -ENODEV;
> >          }
> > -- 
> > 2.1.0
> > 
> > 
>