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>

1 files changed, 2 insertions, 0 deletions

diff --git a/linux-user/main.c b/linux-user/main.c
index 4ddfc9a619..db751c0757 100644
--- a/linux-user/main.c
+++ b/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)