Re: [Qemu-devel] Re: [RFC][PATCH v5 09/21] virtagent: add va.getdmesg RPC

[Michael Roth]

On 12/07/2010 08:37 AM, Jes Sorensen wrote:
> On 12/03/10 19:03, Michael Roth wrote:
> > Add RPC to view guest dmesg output.
> >
> > Signed-off-by: Michael Roth<address@hidden>
> > ---
> >   virtagent-server.c |   46 ++++++++++++++++++++++++++++++++++++++++++++++
> >   1 files changed, 46 insertions(+), 0 deletions(-)
> >
> > diff --git a/virtagent-server.c b/virtagent-server.c
> > index a430b58..aac8f70 100644
> > --- a/virtagent-server.c
> > +++ b/virtagent-server.c
> > @@ -83,6 +83,50 @@ EXIT_CLOSE_BAD:
> >       return result;
> >   }
> >
> > +/* va_getdmesg(): return dmesg output
> > + * rpc return values:
> > + *   - dmesg output as a string
> > + */
> > +static xmlrpc_value *va_getdmesg(xmlrpc_env *env,
> > +                              xmlrpc_value *param,
> > +                              void *user_data)
> > +{
> > +    char *dmesg_buf = NULL, cmd[256];
> > +    int ret;
> > +    xmlrpc_value *result = NULL;
> > +    FILE *pipe;
> > +
> > +    SLOG("va_getdmesg()");
> > +
> > +    dmesg_buf = qemu_mallocz(VA_DMESG_LEN + 2048);
> > +    sprintf(cmd, "dmesg -s %d", VA_DMESG_LEN);
>
> What happens if the guest's dmesg buffer is larger than your hardcoded
> value?

It'll end up getting truncated by the fread() later:

ret = fread(dmesg_buf, sizeof(char), VA_DMESG_LEN, pipe);

That's where the dmesg -s VA_DMESG_LEN comes into play, it should size 
things such that we can buffer up till the end of the dmesg output.

This param is kind of quirky though, size doesn't seem to have an affect 
for anything below 4KB, but if we stick with VA_DMESG_LEN >= 4KB this 
should cover us, unless it's a distro-specific. But it should blow 
anything up, at least.

> Jes