linux-user: Allow gdbstub to ignore page protection

gdbserver ignores page protection by virtue of using /proc/$pid/mem.
Teach qemu gdbstub to do this too. This will not work if /proc is not
mounted; accept this limitation.

One alternative is to temporarily grant the missing PROT_* bit, but
this is inherently racy. Another alternative is self-debugging with
ptrace(POKE), which will break if QEMU itself is being debugged - a
much more severe limitation.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-Id: <20240129093410.3151-2-iii@linux.ibm.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

cpu-target.c

@@ -382,6 +382,9 @@ int cpu_memory_rw_debug(CPUState *cpu, vaddr addr,
     vaddr l, page;
     void * p;
     uint8_t *buf = ptr;
+    ssize_t written;
+    int ret = -1;
+    int fd = -1;
 
     while (len > 0) {
         page = addr & TARGET_PAGE_MASK;
@@ -389,30 +392,75 @@ int cpu_memory_rw_debug(CPUState *cpu, vaddr addr,
         if (l > len)
             l = len;
         flags = page_get_flags(page);
-        if (!(flags & PAGE_VALID))
-            return -1;
+        if (!(flags & PAGE_VALID)) {
+            goto out_close;
+        }
         if (is_write) {
-            if (!(flags & PAGE_WRITE))
-                return -1;
-            /* XXX: this code should not depend on lock_user */
-            if (!(p = lock_user(VERIFY_WRITE, addr, l, 0)))
-                return -1;
-            memcpy(p, buf, l);
-            unlock_user(p, addr, l);
-        } else {
-            if (!(flags & PAGE_READ))
-                return -1;
+            if (flags & PAGE_WRITE) {
+                /* XXX: this code should not depend on lock_user */
+                p = lock_user(VERIFY_WRITE, addr, l, 0);
+                if (!p) {
+                    goto out_close;
+                }
+                memcpy(p, buf, l);
+                unlock_user(p, addr, l);
+            } else {
+                /* Bypass the host page protection using ptrace. */
+                if (fd == -1) {
+                    fd = open("/proc/self/mem", O_WRONLY);
+                    if (fd == -1) {
+                        goto out;
+                    }
+                }
+                /*
+                 * If there is a TranslationBlock and we weren't bypassing the
+                 * host page protection, the memcpy() above would SEGV,
+                 * ultimately leading to page_unprotect(). So invalidate the
+                 * translations manually. Both invalidation and pwrite() must
+                 * be under mmap_lock() in order to prevent the creation of
+                 * another TranslationBlock in between.
+                 */
+                mmap_lock();
+                tb_invalidate_phys_range(addr, addr + l - 1);
+                written = pwrite(fd, buf, l,
+                                 (off_t)(uintptr_t)g2h_untagged(addr));
+                mmap_unlock();
+                if (written != l) {
+                    goto out_close;
+                }
+            }
+        } else if (flags & PAGE_READ) {
             /* XXX: this code should not depend on lock_user */
-            if (!(p = lock_user(VERIFY_READ, addr, l, 1)))
-                return -1;
+            p = lock_user(VERIFY_READ, addr, l, 1);
+            if (!p) {
+                goto out_close;
+            }
             memcpy(buf, p, l);
             unlock_user(p, addr, 0);
+        } else {
+            /* Bypass the host page protection using ptrace. */
+            if (fd == -1) {
+                fd = open("/proc/self/mem", O_RDONLY);
+                if (fd == -1) {
+                    goto out;
+                }
+            }
+            if (pread(fd, buf, l,
+                      (off_t)(uintptr_t)g2h_untagged(addr)) != l) {
+                goto out_close;
+            }
         }
         len -= l;
         buf += l;
         addr += l;
     }
-    return 0;
+    ret = 0;
+out_close:
+    if (fd != -1) {
+        close(fd);
+    }
+out:
+    return ret;
 }
 #endif