linux-user: Add syscall dispatch support

This commit adds support for the `prctl(PR_SET_SYSCALL_USER_DISPATCH)`
function in the Linux userspace emulator.

It is implemented as a fully host-independent function, by forcing
a SIGSYS early during syscall handling, if the PC is outside the
allowed range.

Since disabled SUD is indistinguishable from enabled SUD with
always-allowed region length == ~0, this encoding is used
instead of introducing a new flag.

Tested on [uglendix][1], will probably also apply to software like
tiny-wine, rpcsx, limbo, lazypoline, vicar, sysfail and endokernel,
to name a few.

[1]: https://sr.ht/~arusekk/uglendix

Signed-off-by: Arusekk <floss@arusekk.pl>
Message-ID: <20250711225226.14652-1-floss@arusekk.pl>
[rth: Split out is_vdso_sigreturn region matching and other minor tweaks.]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

linux-user/main.c

@@ -228,6 +228,8 @@ void init_task_state(TaskState *ts)
         ts->start_boottime += bt.tv_nsec * (uint64_t) ticks_per_sec /
                               NANOSECONDS_PER_SECOND;
     }
+
+    ts->sys_dispatch_len = -1;
 }
 
 CPUArchState *cpu_copy(CPUArchState *env)

linux-user/qemu.h

@@ -155,6 +155,11 @@ struct TaskState {
     /* This thread's sigaltstack, if it has one */
     struct target_sigaltstack sigaltstack_used;
 
+    /* This thread's SYSCALL_USER_DISPATCH state, len=~0 means disabled */
+    vaddr sys_dispatch;
+    vaddr sys_dispatch_selector;
+    abi_ulong sys_dispatch_len;
+
     /* Start time of task after system boot in clock ticks */
     uint64_t start_boottime;
 };

linux-user/signal-common.h

@@ -28,6 +28,11 @@ extern abi_ulong default_rt_sigreturn;
 extern abi_ulong vdso_sigreturn_region_start;
 extern abi_ulong vdso_sigreturn_region_end;
 
+static inline bool is_vdso_sigreturn(abi_ulong pc)
+{
+    return pc >= vdso_sigreturn_region_start && pc < vdso_sigreturn_region_end;
+}
+
 void setup_sigtramp(abi_ulong tramp_page);
 
 int on_sig_stack(unsigned long sp);

linux-user/syscall.c

@@ -6344,6 +6344,10 @@ abi_long do_arch_prctl(CPUX86State *env, int code, abi_ulong addr)
 #endif
 #ifndef PR_SET_SYSCALL_USER_DISPATCH
 # define PR_SET_SYSCALL_USER_DISPATCH 59
+# define PR_SYS_DISPATCH_OFF 0
+# define PR_SYS_DISPATCH_ON 1
+# define SYSCALL_DISPATCH_FILTER_ALLOW 0
+# define SYSCALL_DISPATCH_FILTER_BLOCK 1
 #endif
 #ifndef PR_SME_SET_VL
 # define PR_SME_SET_VL  63
@@ -6398,6 +6402,36 @@ static abi_long do_prctl_inval1(CPUArchState *env, abi_long arg2)
 #define do_prctl_sme_set_vl do_prctl_inval1
 #endif
 
+static abi_long do_prctl_syscall_user_dispatch(CPUArchState *env,
+                                               abi_ulong arg2, abi_ulong arg3,
+                                               abi_ulong arg4, abi_ulong arg5)
+{
+    CPUState *cpu = env_cpu(env);
+    TaskState *ts = get_task_state(cpu);
+
+    switch (arg2) {
+    case PR_SYS_DISPATCH_OFF:
+        if (arg3 || arg4 || arg5) {
+            return -TARGET_EINVAL;
+        }
+        ts->sys_dispatch_len = -1;
+        return 0;
+    case PR_SYS_DISPATCH_ON:
+        if (arg3 && arg3 + arg4 <= arg3) {
+            return -TARGET_EINVAL;
+        }
+        if (arg5 && !access_ok(cpu, VERIFY_READ, arg5, 1)) {
+            return -TARGET_EFAULT;
+        }
+        ts->sys_dispatch = arg3;
+        ts->sys_dispatch_len = arg4;
+        ts->sys_dispatch_selector = arg5;
+        return 0;
+    default:
+        return -TARGET_EINVAL;
+    }
+}
+
 static abi_long do_prctl(CPUArchState *env, abi_long option, abi_long arg2,
                          abi_long arg3, abi_long arg4, abi_long arg5)
 {
@@ -6473,6 +6507,9 @@ static abi_long do_prctl(CPUArchState *env, abi_long option, abi_long arg2,
     case PR_SET_UNALIGN:
         return do_prctl_set_unalign(env, arg2);
 
+    case PR_SET_SYSCALL_USER_DISPATCH:
+        return do_prctl_syscall_user_dispatch(env, arg2, arg3, arg4, arg5);
+
     case PR_CAP_AMBIENT:
     case PR_CAPBSET_READ:
     case PR_CAPBSET_DROP:
@@ -6527,7 +6564,6 @@ static abi_long do_prctl(CPUArchState *env, abi_long option, abi_long arg2,
     case PR_SET_MM:
     case PR_GET_SECCOMP:
     case PR_SET_SECCOMP:
-    case PR_SET_SYSCALL_USER_DISPATCH:
     case PR_GET_THP_DISABLE:
     case PR_SET_THP_DISABLE:
     case PR_GET_TSC:
@@ -13897,12 +13933,46 @@ static abi_long do_syscall1(CPUArchState *cpu_env, int num, abi_long arg1,
     return ret;
 }
 
+static bool sys_dispatch(CPUState *cpu, TaskState *ts)
+{
+    abi_ptr pc;
+
+    if (likely(ts->sys_dispatch_len == -1)) {
+        return false;
+    }
+
+    pc = cpu->cc->get_pc(cpu);
+    if (likely(pc - ts->sys_dispatch < ts->sys_dispatch_len)) {
+        return false;
+    }
+    if (unlikely(is_vdso_sigreturn(pc))) {
+        return false;
+    }
+    if (likely(ts->sys_dispatch_selector)) {
+        uint8_t sb;
+        if (get_user_u8(sb, ts->sys_dispatch_selector)) {
+            force_sig(TARGET_SIGSEGV);
+            return true;
+        }
+        if (likely(sb == SYSCALL_DISPATCH_FILTER_ALLOW)) {
+            return false;
+        }
+        if (unlikely(sb != SYSCALL_DISPATCH_FILTER_BLOCK)) {
+            force_sig(TARGET_SIGSYS);
+            return true;
+        }
+    }
+    force_sig_fault(TARGET_SIGSYS, TARGET_SYS_USER_DISPATCH, pc);
+    return true;
+}
+
 abi_long do_syscall(CPUArchState *cpu_env, int num, abi_long arg1,
                     abi_long arg2, abi_long arg3, abi_long arg4,
                     abi_long arg5, abi_long arg6, abi_long arg7,
                     abi_long arg8)
 {
     CPUState *cpu = env_cpu(cpu_env);
+    TaskState *ts = get_task_state(cpu);
     abi_long ret;
 
 #ifdef DEBUG_ERESTARTSYS
@@ -13919,6 +13989,10 @@ abi_long do_syscall(CPUArchState *cpu_env, int num, abi_long arg1,
     }
 #endif
 
+    if (sys_dispatch(cpu, ts)) {
+        return -QEMU_ESIGRETURN;
+    }
+
     record_syscall_start(cpu, num, arg1,
                          arg2, arg3, arg4, arg5, arg6, arg7, arg8);
 

linux-user/syscall_defs.h

@@ -689,6 +689,12 @@ typedef struct target_siginfo {
 #define TARGET_TRAP_HWBKPT      (4)     /* hardware breakpoint/watchpoint */
 #define TARGET_TRAP_UNK         (5)     /* undiagnosed trap */
 
+/*
+ * SIGSYS si_codes
+ */
+#define TARGET_SYS_SECCOMP       (1)  /* seccomp triggered */
+#define TARGET_SYS_USER_DISPATCH (2)  /* syscall user dispatch triggered */
+
 /*
  * SIGEMT si_codes
  */