linux-user: Use `qemu_log' for strace

This change switches linux-user strace logging to use the newer `qemu_log`
logging subsystem rather than the older `gemu_log` (notice the "g")
logger. `qemu_log` has several advantages, namely that it allows logging
to a file, and provides a more unified interface for configuration
of logging (via the QEMU_LOG environment variable or options).

This change introduces a new log mask: `LOG_STRACE` which is used for
logging of user-mode strace messages.

Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Josh Kunz <jkz@google.com>
Message-Id: <20200204025416.111409-3-jkz@google.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

include/qemu/log.h

@@ -62,6 +62,8 @@ static inline bool qemu_log_separate(void)
 #define CPU_LOG_TB_OP_IND  (1 << 16)
 #define CPU_LOG_TB_FPU     (1 << 17)
 #define CPU_LOG_PLUGIN     (1 << 18)
+/* LOG_STRACE is used for user-mode strace logging. */
+#define LOG_STRACE         (1 << 19)
 
 /* Lock output for a series of related logs.  Since this is not needed
  * for a single qemu_log / qemu_log_mask / qemu_log_mask_and_addr, we

linux-user/main.c

@@ -60,6 +60,19 @@ unsigned long mmap_min_addr;
 unsigned long guest_base;
 int have_guest_base;
 
+/*
+ * Used to implement backwards-compatibility for the `-strace`, and
+ * QEMU_STRACE options. Without this, the QEMU_LOG can be overwritten by
+ * -strace, or vice versa.
+ */
+static bool enable_strace;
+
+/*
+ * The last log mask given by the user in an environment variable or argument.
+ * Used to support command line arguments overriding environment variables.
+ */
+static int last_log_mask;
+
 /*
  * When running 32-on-64 we should make sure we can fit all of the possible
  * guest address space into a contiguous chunk of virtual host memory.
@@ -223,15 +236,11 @@ static void handle_arg_help(const char *arg)
 
 static void handle_arg_log(const char *arg)
 {
-    int mask;
-
-    mask = qemu_str_to_log_mask(arg);
-    if (!mask) {
+    last_log_mask = qemu_str_to_log_mask(arg);
+    if (!last_log_mask) {
         qemu_print_log_usage(stdout);
         exit(EXIT_FAILURE);
     }
-    qemu_log_needs_buffers();
-    qemu_set_log(mask);
 }
 
 static void handle_arg_dfilter(const char *arg)
@@ -375,7 +384,7 @@ static void handle_arg_singlestep(const char *arg)
 
 static void handle_arg_strace(const char *arg)
 {
-    do_strace = 1;
+    enable_strace = true;
 }
 
 static void handle_arg_version(const char *arg)
@@ -629,6 +638,7 @@ int main(int argc, char **argv, char **envp)
     int i;
     int ret;
     int execfd;
+    int log_mask;
     unsigned long max_reserved_va;
 
     error_init(argv[0]);
@@ -661,6 +671,12 @@ int main(int argc, char **argv, char **envp)
 
     optind = parse_args(argc, argv);
 
+    log_mask = last_log_mask | (enable_strace ? LOG_STRACE : 0);
+    if (log_mask) {
+        qemu_log_needs_buffers();
+        qemu_set_log(log_mask);
+    }
+
     if (!trace_init_backends()) {
         exit(1);
     }

linux-user/qemu.h

@@ -386,7 +386,6 @@ void print_syscall_ret(int num, abi_long arg1);
  * --- SIGSEGV {si_signo=SIGSEGV, si_code=SI_KERNEL, si_addr=0} ---
  */
 void print_taken_signal(int target_signum, const target_siginfo_t *tinfo);
-extern int do_strace;
 
 /* signal.c */
 void process_pending_signals(CPUArchState *cpu_env);

linux-user/signal.c

@@ -934,7 +934,7 @@ static void handle_pending_signal(CPUArchState *cpu_env, int sig,
         handler = sa->_sa_handler;
     }
 
-    if (do_strace) {
+    if (unlikely(qemu_loglevel_mask(LOG_STRACE))) {
         print_taken_signal(sig, &k->info);
     }
 

linux-user/syscall.c

@@ -12171,14 +12171,15 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
     record_syscall_start(cpu, num, arg1,
                          arg2, arg3, arg4, arg5, arg6, arg7, arg8);
 
-    if (unlikely(do_strace)) {
+    if (unlikely(qemu_loglevel_mask(LOG_STRACE))) {
         print_syscall(num, arg1, arg2, arg3, arg4, arg5, arg6);
-        ret = do_syscall1(cpu_env, num, arg1, arg2, arg3, arg4,
-                          arg5, arg6, arg7, arg8);
+    }
+
+    ret = do_syscall1(cpu_env, num, arg1, arg2, arg3, arg4,
+                      arg5, arg6, arg7, arg8);
+
+    if (unlikely(qemu_loglevel_mask(LOG_STRACE))) {
         print_syscall_ret(num, ret);
-    } else {
-        ret = do_syscall1(cpu_env, num, arg1, arg2, arg3, arg4,
-                          arg5, arg6, arg7, arg8);
     }
 
     record_syscall_return(cpu, num, ret);

util/log.c

@@ -332,6 +332,8 @@ const QEMULogItem qemu_log_items[] = {
 #ifdef CONFIG_PLUGIN
     { CPU_LOG_PLUGIN, "plugin", "output from TCG plugins\n"},
 #endif
+    { LOG_STRACE, "strace",
+      "log every user-mode syscall, its input, and its result" },
     { 0, NULL, NULL },
 };