[Qemu-devel] [PATCH] Overwrite argv to set process title, eliminating 16-character prctl() limit.

[John Morrissey]

Linux seems to maintain the length of the original args, even when the new
args are shorter and NULL-terminated, so the trailing whitespace in ps(1)
output is probably unavoidable. I've seen the same result with other daemons
that overwrite argv.

Keeps the call to prctl(), since some tools get the process name from
/proc/PID/status.

Signed-off-by: John Morrissey <address@hidden>
---
 os-posix.c      |   34 +++++++++++++++++++++-------------
 qemu-os-posix.h |    2 +-
 qemu-os-win32.h |    3 ++-
 vl.c            |   11 +++++++++--
 4 files changed, 33 insertions(+), 17 deletions(-)

diff --git a/os-posix.c b/os-posix.c
index 38c29d1..04cc2e1 100644
--- a/os-posix.c
+++ b/os-posix.c
@@ -39,10 +39,6 @@
 #include "net/slirp.h"
 #include "qemu-options.h"
 
-#ifdef CONFIG_LINUX
-#include <sys/prctl.h>
-#endif
-
 #ifdef CONFIG_EVENTFD
 #include <sys/eventfd.h>
 #endif
@@ -149,20 +145,32 @@ char *os_find_datadir(const char *argv0)
 #undef SHARE_SUFFIX
 #undef BUILD_SUFFIX
 
-void os_set_proc_name(const char *s)
+void os_set_proc_name(int argc, char **argv, const char *name)
 {
-#if defined(PR_SET_NAME)
-    char name[16];
-    if (!s)
+#ifdef CONFIG_LINUX
+    char prctl_name[16], *last_argv_byte, *p;
+    int len, i;
+
+    if (!name)
         return;
-    name[sizeof(name) - 1] = 0;
-    strncpy(name, s, sizeof(name));
-    /* Could rewrite argv[0] too, but that's a bit more complicated.
-       This simple way is enough for `top'. */
-    if (prctl(PR_SET_NAME, name)) {
+
+#if defined(PR_SET_NAME)
+    strncpy(prctl_name, name, sizeof(prctl_name));
+    if (prctl(PR_SET_NAME, prctl_name)) {
         perror("unable to change process name");
         exit(1);
     }
+#endif
+
+    last_argv_byte = argv[argc - 1] + strlen(argv[argc - 1]);
+
+    len = snprintf(argv[0], last_argv_byte - argv[0], "%s", name);
+
+    p = &argv[0][len];
+    while (p <= last_argv_byte)
+        *p++ = '\0';
+    for (i = 1; i < argc; ++i)
+        argv[i] = (char *) "";
 #else
     fprintf(stderr, "Change of process name not supported by your OS\n");
     exit(1);
diff --git a/qemu-os-posix.h b/qemu-os-posix.h
index 353f878..b0cf993 100644
--- a/qemu-os-posix.h
+++ b/qemu-os-posix.h
@@ -31,7 +31,7 @@ static inline void os_host_main_loop_wait(int *timeout)
 }
 
 void os_set_line_buffering(void);
-void os_set_proc_name(const char *s);
+void os_set_proc_name(int argc, char **argv, const char *name);
 void os_setup_signal_handling(void);
 void os_daemonize(void);
 void os_setup_post(void);
diff --git a/qemu-os-win32.h b/qemu-os-win32.h
index 1a07e5e..c618362 100644
--- a/qemu-os-win32.h
+++ b/qemu-os-win32.h
@@ -46,7 +46,8 @@ static inline void os_setup_signal_handling(void) {}
 static inline void os_daemonize(void) {}
 static inline void os_setup_post(void) {}
 void os_set_line_buffering(void);
-static inline void os_set_proc_name(const char *dummy) {}
+static inline void os_set_proc_name(int argc, char **argv,
+                                    const char *dummy) {}
 
 #if !defined(EPROTONOSUPPORT)
 # define EPROTONOSUPPORT EINVAL
diff --git a/vl.c b/vl.c
index 805e11f..74a939c 100644
--- a/vl.c
+++ b/vl.c
@@ -1776,7 +1776,11 @@ static const QEMUOption *lookup_opt(int argc, char 
**argv,
         optarg = NULL;
     }
 
-    *poptarg = optarg;
+    if (optarg != NULL) {
+        *poptarg = qemu_strdup(optarg);
+    } else {
+        *poptarg = NULL;
+    }
     *poptind = optind;
 
     return popt;
@@ -1804,6 +1808,7 @@ int main(int argc, char **argv, char **envp)
     int tb_size;
     const char *pid_file = NULL;
     const char *incoming = NULL;
+    const char *process_name = NULL;
     int show_vnc_port = 0;
     int defconfig = 1;
 
@@ -2497,7 +2502,7 @@ int main(int argc, char **argv, char **envp)
                            exit(1);
                        }
                        p += 8;
-                       os_set_proc_name(p);
+                       process_name = p;
                     }  
                 }      
                 break;
@@ -2723,6 +2728,8 @@ int main(int argc, char **argv, char **envp)
         exit(1);
     }
 
+    os_set_proc_name(argc, argv, process_name);
+
     if (kvm_allowed) {
         int ret = kvm_init(smp_cpus);
         if (ret < 0) {
-- 
1.7.0.4