Re: [Qemu-devel] [PATCH] log: Redirect stderr to logfile if deamonized

[Dimitris Aragiorgis <dimara@arrikto.com>]

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* Paolo Bonzini <pbonzini@redhat.com> [2016-02-18 16:22:13 +0100]:

> 
> 
> On 18/02/2016 12:38, Dimitris Aragiorgis wrote:
> > In case of daemonize, use the logfile passed with the -D option in
> > order to redirect stderr to it instead of /dev/null.
> > 
> > Also remove some unused code in log.h.
> > 
> > Signed-off-by: Dimitris Aragiorgis <dimara@arrikto.com>
> > ---
> >  include/qemu/log.h |    6 ------
> >  os-posix.c         |    6 +++++-
> >  util/log.c         |   11 +++++++++--
> >  3 files changed, 14 insertions(+), 9 deletions(-)
> > 
> > diff --git a/include/qemu/log.h b/include/qemu/log.h
> > index 30817f7..dda65fd 100644
> > --- a/include/qemu/log.h
> > +++ b/include/qemu/log.h
> > @@ -94,12 +94,6 @@ static inline void qemu_log_close(void)
> >      }
> >  }
> >  
> > -/* Set up a new log file */
> > -static inline void qemu_log_set_file(FILE *f)
> > -{
> > -    qemu_logfile = f;
> > -}
> > -
> >  /* define log items */
> >  typedef struct QEMULogItem {
> >      int mask;
> > diff --git a/os-posix.c b/os-posix.c
> > index cce62ed..92fa3ba 100644
> > --- a/os-posix.c
> > +++ b/os-posix.c
> > @@ -37,6 +37,7 @@
> >  #include "qemu-options.h"
> >  #include "qemu/rcu.h"
> >  #include "qemu/error-report.h"
> > +#include "qemu/log.h"
> >  
> >  #ifdef CONFIG_LINUX
> >  #include <sys/prctl.h>
> > @@ -275,7 +276,10 @@ void os_setup_post(void)
> >  
> >          dup2(fd, 0);
> >          dup2(fd, 1);
> > -        dup2(fd, 2);
> > +        /* In case -D is given do not redirect stderr to /dev/null */
> > +        if (!qemu_logfile) {
> > +            dup2(fd, 2);
> > +        }
> >  
> >          close(fd);
> >  
> > diff --git a/util/log.c b/util/log.c
> > index 2709e98..a7ddc7e 100644
> > --- a/util/log.c
> > +++ b/util/log.c
> > @@ -56,13 +56,20 @@ void do_qemu_set_log(int log_flags, bool use_own_buffers)
> >  #ifdef CONFIG_TRACE_LOG
> >      qemu_loglevel |= LOG_TRACE;
> >  #endif
> > -    if (qemu_loglevel && !qemu_logfile) {
> > +    if ((qemu_loglevel || is_daemonized()) && !qemu_logfile) {
> >          if (logfilename) {
> >              qemu_logfile = fopen(logfilename, log_append ? "a" : "w");
> >              if (!qemu_logfile) {
> >                  perror(logfilename);
> >                  _exit(1);
> >              }
> > +            /* In case we are a daemon redirect stderr to logfile */
> > +            if (is_daemonized()) {
> > +                dup2(fileno(qemu_logfile), STDERR_FILENO);
> > +                fclose(qemu_logfile);
> > +                /* This will skip closing logfile in qemu_log_close() */
> > +                qemu_logfile = stderr;
> > +            }
> >          } else {
> >              /* Default to stderr if no log file specified */
> >              qemu_logfile = stderr;
> > @@ -82,7 +89,7 @@ void do_qemu_set_log(int log_flags, bool use_own_buffers)
> >              log_append = 1;
> >          }
> >      }
> > -    if (!qemu_loglevel && qemu_logfile) {
> > +    if (!qemu_loglevel && !is_daemonized() && qemu_logfile) {
> >          qemu_log_close();
> >      }
> 
> Why is this necessary?  Perhaps qemu_log_close should dup(1,2) if QEMU
> is daemonized.  The rest looks great.
> 

Without !is_daemonized(), if we use -daemon with -D without -d,
qemu_log_close() will eventually set qemu_logfile to NULL. This
will make os_setup_post() redirect stderr to /dev/null, which
is unwanted.

To be honest, I don't understand your suggestion. How would calling
dup2(1, 2) help in our case? Isn't fd 1 the standard output?

Thanks,
dimara

> Paolo

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