[PATCH v2 09/11] linux-user: Make the qemu detection for /proc/$pid/exe a bit conservative

[YAMAMOTO Takashi]

Perform the qemu special case only when the binary seems the same as
our own executable.
This is enough for my use case (docker and runc) where the involved
qemu binaries are always for the same arch.

Signed-off-by: YAMAMOTO Takashi <yamamoto@midokura.com>
---
 linux-user/syscall.c | 38 ++++++++++++++++++++++++++++++++++----
 1 file changed, 34 insertions(+), 4 deletions(-)

diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 6f9161dbe4..56a3c37d83 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -7986,6 +7986,26 @@ static int open_self_auxv(void *cpu_env, int fd)
     return 0;
 }
 
+static int is_same_qemu(const char *their_exe)
+{
+    struct stat our_st;
+    struct stat their_st;
+
+    if (stat("/proc/self/exe", &our_st) != 0) {
+        return 0;
+    }
+    if (stat(their_exe, &their_st) != 0) {
+        return 0;
+    }
+    if (our_st.st_dev != their_st.st_dev) {
+        return 0;
+    }
+    if (our_st.st_ino != their_st.st_ino) {
+        return 0;
+    }
+    return 1;
+}
+
 static const char *get_exe_path(int pid, char *buf, size_t bufsize)
 {
     ssize_t ssz;
@@ -7996,6 +8016,20 @@ static const char *get_exe_path(int pid, char *buf, 
size_t bufsize)
     }
 
     /* dockerd makes runc invoke dockerd using "/proc/${dockerd_pid}/exe". */
+
+    /*
+     * Check that it's the same qemu binary as ours
+     * to avoid false positives.
+     *
+     * While ideally we want to allow different qemu binaries,
+     * (E.g. linux-user for a different arch)
+     * I can't think of any reliable way to detect the cases.
+     */
+    snprintf(buf, bufsize, "/proc/%d/exe", pid);
+    if (!is_same_qemu(buf)) {
+        return NULL;
+    }
+
     snprintf(buf, bufsize, "/proc/%d/cmdline", pid);
     fd = open(buf, O_RDONLY);
     if (fd == -1) {
@@ -8033,10 +8067,6 @@ static const char *get_exe_path(int pid, char *buf, 
size_t bufsize)
             }
         }
 
-        /*
-         * XXX a bit too loose detection of qemu.
-         * maybe we can compare /proc/$pid/exe with ours.
-         */
         slash = strrchr(argv0, '/');
         if (slash != NULL) {
             argv0 = slash + 1; /* basename */
-- 
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