bsd-user: Handle short reads in mmap_h_gt_g

In particular, if an image has a large bss, we can hit EOF before reading
all bytes of the mapping.  Mirror the similar change to linux-user.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20240820050848.165253-3-richard.henderson@linaro.org>

bsd-user/mmap.c

@@ -128,6 +128,40 @@ error:
     return ret;
 }
 
+/*
+ * Perform a pread on behalf of target_mmap.  We can reach EOF, we can be
+ * interrupted by signals, and in general there's no good error return path.
+ * If @zero, zero the rest of the block at EOF.
+ * Return true on success.
+ */
+static bool mmap_pread(int fd, void *p, size_t len, off_t offset, bool zero)
+{
+    while (1) {
+        ssize_t r = pread(fd, p, len, offset);
+
+        if (likely(r == len)) {
+            /* Complete */
+            return true;
+        }
+        if (r == 0) {
+            /* EOF */
+            if (zero) {
+                memset(p, 0, len);
+            }
+            return true;
+        }
+        if (r > 0) {
+            /* Short read */
+            p += r;
+            len -= r;
+            offset += r;
+        } else if (errno != EINTR) {
+            /* Error */
+            return false;
+        }
+    }
+}
+
 /*
  * map an incomplete host page
  *
@@ -190,7 +224,7 @@ static int mmap_frag(abi_ulong real_start,
             mprotect(host_start, qemu_host_page_size, prot1 | PROT_WRITE);
 
         /* read the corresponding file data */
-        if (pread(fd, g2h_untagged(start), end - start, offset) == -1) {
+        if (!mmap_pread(fd, g2h_untagged(start), end - start, offset, true)) {
             return -1;
         }
 
@@ -565,7 +599,7 @@ abi_long target_mmap(abi_ulong start, abi_ulong len, int prot,
                                   -1, 0);
             if (retaddr == -1)
                 goto fail;
-            if (pread(fd, g2h_untagged(start), len, offset) == -1) {
+            if (!mmap_pread(fd, g2h_untagged(start), len, offset, false)) {
                 goto fail;
             }
             if (!(prot & PROT_WRITE)) {