linux-user: Explicitly untag memory management syscalls

We define target_mmap et al as untagged, so that they can be
used from the binary loaders.  Explicitly call cpu_untagged_addr
for munmap, mprotect, mremap syscall entry points.

Add a few comments for the syscalls that are exempted by the
kernel's tagged-address-abi.rst.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210212184902.1251044-14-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

linux-user/syscall.c

@@ -894,6 +894,8 @@ abi_long do_brk(abi_ulong new_brk)
     abi_long mapped_addr;
     abi_ulong new_alloc_size;
 
+    /* brk pointers are always untagged */
+
     DEBUGF_BRK("do_brk(" TARGET_ABI_FMT_lx ") -> ", new_brk);
 
     if (!new_brk) {
@@ -4599,6 +4601,8 @@ static inline abi_ulong do_shmat(CPUArchState *cpu_env,
     int i,ret;
     abi_ulong shmlba;
 
+    /* shmat pointers are always untagged */
+
     /* find out the length of the shared memory segment */
     ret = get_errno(shmctl(shmid, IPC_STAT, &shm_info));
     if (is_error(ret)) {
@@ -4666,6 +4670,8 @@ static inline abi_long do_shmdt(abi_ulong shmaddr)
     int i;
     abi_long rv;
 
+    /* shmdt pointers are always untagged */
+
     mmap_lock();
 
     for (i = 0; i < N_SHM_REGIONS; ++i) {
@@ -9703,6 +9709,7 @@ static abi_long do_syscall1(void *cpu_env, int num, abi_long arg1,
                                         v5, v6));
         }
 #else
+        /* mmap pointers are always untagged */
         ret = get_errno(target_mmap(arg1, arg2, arg3,
                                     target_to_host_bitmask(arg4, mmap_flags_tbl),
                                     arg5,
@@ -9721,8 +9728,10 @@ static abi_long do_syscall1(void *cpu_env, int num, abi_long arg1,
         return get_errno(ret);
 #endif
     case TARGET_NR_munmap:
+        arg1 = cpu_untagged_addr(cpu, arg1);
         return get_errno(target_munmap(arg1, arg2));
     case TARGET_NR_mprotect:
+        arg1 = cpu_untagged_addr(cpu, arg1);
         {
             TaskState *ts = cpu->opaque;
             /* Special hack to detect libc making the stack executable.  */
@@ -9737,6 +9746,8 @@ static abi_long do_syscall1(void *cpu_env, int num, abi_long arg1,
         return get_errno(target_mprotect(arg1, arg2, arg3));
 #ifdef TARGET_NR_mremap
     case TARGET_NR_mremap:
+        arg1 = cpu_untagged_addr(cpu, arg1);
+        /* mremap new_addr (arg5) is always untagged */
         return get_errno(target_mremap(arg1, arg2, arg3, arg4, arg5));
 #endif
         /* ??? msync/mlock/munlock are broken for softmmu.  */