bsd-user: Implement do_obreak function

Match linux-user, by manually applying the following commits, in order:

d28b3c90   linux-user: Make sure initial brk(0) is page-aligned
15ad9853   linux-user: Fix qemu brk() to not zero bytes on current page
dfe49864   linux-user: Prohibit brk() to to shrink below initial heap address
eac78a4b   linux-user: Fix signed math overflow in brk() syscall
c6cc059e   linux-user: Do not call get_errno() in do_brk()
e69e032d   linux-user: Use MAP_FIXED_NOREPLACE for do_brk()
cb9d5d1f   linux-user: Do nothing if too small brk is specified
2aea137a   linux-user: Do not align brk with host page size

Signed-off-by: Stacey Son <sson@FreeBSD.org>
Signed-off-by: Karim Taha <kariem.taha2.7@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230925182709.4834-19-kariem.taha2.7@gmail.com>

bsd-user/bsd-mem.h

@@ -212,4 +212,49 @@ static inline abi_long do_bsd_mincore(abi_ulong target_addr, abi_ulong len,
     return ret;
 }
 
+/* do_brk() must return target values and target errnos. */
+static inline abi_long do_obreak(abi_ulong brk_val)
+{
+    abi_long mapped_addr;
+    abi_ulong new_brk;
+    abi_ulong old_brk;
+
+    /* brk pointers are always untagged */
+
+    /* do not allow to shrink below initial brk value */
+    if (brk_val < initial_target_brk) {
+        return target_brk;
+    }
+
+    new_brk = TARGET_PAGE_ALIGN(brk_val);
+    old_brk = TARGET_PAGE_ALIGN(target_brk);
+
+    /* new and old target_brk might be on the same page */
+    if (new_brk == old_brk) {
+        target_brk = brk_val;
+        return target_brk;
+    }
+
+    /* Release heap if necesary */
+    if (new_brk < old_brk) {
+        target_munmap(new_brk, old_brk - new_brk);
+
+        target_brk = brk_val;
+        return target_brk;
+    }
+
+    mapped_addr = target_mmap(old_brk, new_brk - old_brk,
+                              PROT_READ | PROT_WRITE,
+                              MAP_FIXED | MAP_EXCL | MAP_ANON | MAP_PRIVATE,
+                              -1, 0);
+
+    if (mapped_addr == old_brk) {
+        target_brk = brk_val;
+        return target_brk;
+    }
+
+    /* For everything else, return the previous break. */
+    return target_brk;
+}
+
 #endif /* BSD_USER_BSD_MEM_H */

bsd-user/freebsd/os-syscall.c

@@ -855,6 +855,13 @@ static abi_long freebsd_syscall(void *cpu_env, int num, abi_long arg1,
         break;
 #endif
 
+        /*
+         * Misc
+         */
+    case TARGET_FREEBSD_NR_break:
+        ret = do_obreak(arg1);
+        break;
+
         /*
          * sys{ctl, arch, call}
          */