[PULL,26/37] linux-user: Don't assume 0 is not a valid host timer_t value

Commit Message

Laurent Vivier Sept. 28, 2022, 8:27 p.m. UTC

From: Peter Maydell <[e-mail]>

For handling guest POSIX timers, we currently use an array
g_posix_timers[], whose entries are a host timer_t value, or 0 for
"this slot is unused".  When the guest calls the timer_create syscall
we look through the array for a slot containing 0, and use that for
the new timer.

This scheme assumes that host timer_t values can never be zero.  This
is unfortunately not a valid assumption -- for some host libc
versions, timer_t values are simply indexes starting at 0.  When
using this kind of host libc, the effect is that the first and second
timers end up sharing a slot, and so when the guest tries to operate
on the first timer it changes the second timer instead.

Rework the timer allocation code, so that:
 * the 'slot in use' indication uses a separate array from the
   host timer_t array
 * we grab the free slot atomically, to avoid races when multiple
   threads call timer_create simultaneously
 * releasing an allocated slot is abstracted out into a new
   free_host_timer_slot() function called in the correct places

This fixes:
 * problems on hosts where timer_t 0 is valid
 * the FIXME in next_free_host_timer() about locking
 * bugs in the error paths in timer_create where we forgot to release
   the slot we grabbed, or forgot to free the host timer

Reported-by: Jon Alduan <[e-mail]>
Signed-off-by: Peter Maydell <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Laurent Vivier <[e-mail]>
---
 linux-user/syscall.c | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

Patch

diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 54b29f3b406a..e0e0f058121f 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -525,20 +525,25 @@  _syscall4(int, sys_prlimit64, pid_t, pid, int, resource,
 
 #if defined(TARGET_NR_timer_create)
 /* Maximum of 32 active POSIX timers allowed at any one time. */
-static timer_t g_posix_timers[32] = { 0, } ;
+#define GUEST_TIMER_MAX 32
+static timer_t g_posix_timers[GUEST_TIMER_MAX];
+static int g_posix_timer_allocated[GUEST_TIMER_MAX];
 
 static inline int next_free_host_timer(void)
 {
-    int k ;
-    /* FIXME: Does finding the next free slot require a lock? */
-    for (k = 0; k < ARRAY_SIZE(g_posix_timers); k++) {
-        if (g_posix_timers[k] == 0) {
-            g_posix_timers[k] = (timer_t) 1;
+    int k;
+    for (k = 0; k < ARRAY_SIZE(g_posix_timer_allocated); k++) {
+        if (qatomic_xchg(g_posix_timer_allocated + k, 1) == 0) {
             return k;
         }
     }
     return -1;
 }
+
+static inline void free_host_timer_slot(int id)
+{
+    qatomic_store_release(g_posix_timer_allocated + id, 0);
+}
 #endif
 
 static inline int host_to_target_errno(int host_errno)
@@ -12896,15 +12901,18 @@  static abi_long do_syscall1(CPUArchState *cpu_env, int num, abi_long arg1,
                 phost_sevp = &host_sevp;
                 ret = target_to_host_sigevent(phost_sevp, arg2);
                 if (ret != 0) {
+                    free_host_timer_slot(timer_index);
                     return ret;
                 }
             }
 
             ret = get_errno(timer_create(clkid, phost_sevp, phtimer));
             if (ret) {
-                phtimer = NULL;
+                free_host_timer_slot(timer_index);
             } else {
                 if (put_user(TIMER_MAGIC | timer_index, arg3, target_timer_t)) {
+                    timer_delete(*phtimer);
+                    free_host_timer_slot(timer_index);
                     return -TARGET_EFAULT;
                 }
             }
@@ -13040,7 +13048,7 @@  static abi_long do_syscall1(CPUArchState *cpu_env, int num, abi_long arg1,
         } else {
             timer_t htimer = g_posix_timers[timerid];
             ret = get_errno(timer_delete(htimer));
-            g_posix_timers[timerid] = 0;
+            free_host_timer_slot(timerid);
         }
         return ret;
     }