memory: add readonly support to memory_region_init_ram_from_file()

There is currently no way to open(O_RDONLY) and mmap(PROT_READ) when
creating a memory region from a file. This functionality is needed since
the underlying host file may not allow writing.

Add a bool readonly argument to memory_region_init_ram_from_file() and
the APIs it calls.

Extend memory_region_init_ram_from_file() rather than introducing a
memory_region_init_rom_from_file() API so that callers can easily make a
choice between read/write and read-only at runtime without calling
different APIs.

No new RAMBlock flag is introduced for read-only because it's unclear
whether RAMBlocks need to know that they are read-only. Pass a bool
readonly argument instead.

Both of these design decisions can be changed in the future. It just
seemed like the simplest approach to me.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20210104171320.575838-2-stefanha@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

backends/hostmem-file.c

@@ -56,7 +56,7 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
                                      backend->size, fb->align,
                                      (backend->share ? RAM_SHARED : 0) |
                                      (fb->is_pmem ? RAM_PMEM : 0),
-                                     fb->mem_path, errp);
+                                     fb->mem_path, false, errp);
     g_free(name);
 #endif
 }

include/exec/memory.h

@@ -966,6 +966,7 @@ void memory_region_init_resizeable_ram(MemoryRegion *mr,
  *             - RAM_PMEM: the memory is persistent memory
  *             Other bits are ignored now.
  * @path: the path in which to allocate the RAM.
+ * @readonly: true to open @path for reading, false for read/write.
  * @errp: pointer to Error*, to store an error if it happens.
  *
  * Note that this function does not do anything to cause the data in the
@@ -978,6 +979,7 @@ void memory_region_init_ram_from_file(MemoryRegion *mr,
                                       uint64_t align,
                                       uint32_t ram_flags,
                                       const char *path,
+                                      bool readonly,
                                       Error **errp);
 
 /**

include/exec/ram_addr.h

@@ -110,6 +110,7 @@ long qemu_maxrampagesize(void);
  *              - RAM_PMEM: the backend @mem_path or @fd is persistent memory
  *              Other bits are ignored.
  *  @mem_path or @fd: specify the backing file or device
+ *  @readonly: true to open @path for reading, false for read/write.
  *  @errp: pointer to Error*, to store an error if it happens
  *
  * Return:
@@ -118,9 +119,9 @@ long qemu_maxrampagesize(void);
  */
 RAMBlock *qemu_ram_alloc_from_file(ram_addr_t size, MemoryRegion *mr,
                                    uint32_t ram_flags, const char *mem_path,
-                                   Error **errp);
+                                   bool readonly, Error **errp);
 RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
-                                 uint32_t ram_flags, int fd,
+                                 uint32_t ram_flags, int fd, bool readonly,
                                  Error **errp);
 
 RAMBlock *qemu_ram_alloc_from_ptr(ram_addr_t size, void *host,

include/qemu/mmap-alloc.h

@@ -14,6 +14,7 @@ size_t qemu_mempath_getpagesize(const char *mem_path);
  *  @size: the number of bytes to be mmaped
  *  @align: if not zero, specify the alignment of the starting mapping address;
  *          otherwise, the alignment in use will be determined by QEMU.
+ *  @readonly: true for a read-only mapping, false for read/write.
  *  @shared: map has RAM_SHARED flag.
  *  @is_pmem: map has RAM_PMEM flag.
  *
@@ -24,6 +25,7 @@ size_t qemu_mempath_getpagesize(const char *mem_path);
 void *qemu_ram_mmap(int fd,
                     size_t size,
                     size_t align,
+                    bool readonly,
                     bool shared,
                     bool is_pmem);
 

softmmu/memory.c

@@ -1587,15 +1587,18 @@ void memory_region_init_ram_from_file(MemoryRegion *mr,
                                       uint64_t align,
                                       uint32_t ram_flags,
                                       const char *path,
+                                      bool readonly,
                                       Error **errp)
 {
     Error *err = NULL;
     memory_region_init(mr, owner, name, size);
     mr->ram = true;
+    mr->readonly = readonly;
     mr->terminates = true;
     mr->destructor = memory_region_destructor_ram;
     mr->align = align;
-    mr->ram_block = qemu_ram_alloc_from_file(size, mr, ram_flags, path, &err);
+    mr->ram_block = qemu_ram_alloc_from_file(size, mr, ram_flags, path,
+                                             readonly, &err);
     if (err) {
         mr->size = int128_zero();
         object_unparent(OBJECT(mr));
@@ -1618,7 +1621,7 @@ void memory_region_init_ram_from_fd(MemoryRegion *mr,
     mr->destructor = memory_region_destructor_ram;
     mr->ram_block = qemu_ram_alloc_from_fd(size, mr,
                                            share ? RAM_SHARED : 0,
-                                           fd, &err);
+                                           fd, false, &err);
     if (err) {
         mr->size = int128_zero();
         object_unparent(OBJECT(mr));

softmmu/physmem.c

@@ -1398,6 +1398,7 @@ static int64_t get_file_align(int fd)
 
 static int file_ram_open(const char *path,
                          const char *region_name,
+                         bool readonly,
                          bool *created,
                          Error **errp)
 {
@@ -1408,7 +1409,7 @@ static int file_ram_open(const char *path,
 
     *created = false;
     for (;;) {
-        fd = open(path, O_RDWR);
+        fd = open(path, readonly ? O_RDONLY : O_RDWR);
         if (fd >= 0) {
             /* @path names an existing file, use it */
             break;
@@ -1460,6 +1461,7 @@ static int file_ram_open(const char *path,
 static void *file_ram_alloc(RAMBlock *block,
                             ram_addr_t memory,
                             int fd,
+                            bool readonly,
                             bool truncate,
                             Error **errp)
 {
@@ -1510,7 +1512,7 @@ static void *file_ram_alloc(RAMBlock *block,
         perror("ftruncate");
     }
 
-    area = qemu_ram_mmap(fd, memory, block->mr->align,
+    area = qemu_ram_mmap(fd, memory, block->mr->align, readonly,
                          block->flags & RAM_SHARED, block->flags & RAM_PMEM);
     if (area == MAP_FAILED) {
         error_setg_errno(errp, errno,
@@ -1942,7 +1944,7 @@ static void ram_block_add(RAMBlock *new_block, Error **errp, bool shared)
 
 #ifdef CONFIG_POSIX
 RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
-                                 uint32_t ram_flags, int fd,
+                                 uint32_t ram_flags, int fd, bool readonly,
                                  Error **errp)
 {
     RAMBlock *new_block;
@@ -1996,7 +1998,8 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
     new_block->used_length = size;
     new_block->max_length = size;
     new_block->flags = ram_flags;
-    new_block->host = file_ram_alloc(new_block, size, fd, !file_size, errp);
+    new_block->host = file_ram_alloc(new_block, size, fd, readonly,
+                                     !file_size, errp);
     if (!new_block->host) {
         g_free(new_block);
         return NULL;
@@ -2015,18 +2018,19 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
 
 RAMBlock *qemu_ram_alloc_from_file(ram_addr_t size, MemoryRegion *mr,
                                    uint32_t ram_flags, const char *mem_path,
-                                   Error **errp)
+                                   bool readonly, Error **errp)
 {
     int fd;
     bool created;
     RAMBlock *block;
 
-    fd = file_ram_open(mem_path, memory_region_name(mr), &created, errp);
+    fd = file_ram_open(mem_path, memory_region_name(mr), readonly, &created,
+                       errp);
     if (fd < 0) {
         return NULL;
     }
 
-    block = qemu_ram_alloc_from_fd(size, mr, ram_flags, fd, errp);
+    block = qemu_ram_alloc_from_fd(size, mr, ram_flags, fd, readonly, errp);
     if (!block) {
         if (created) {
             unlink(mem_path);

util/mmap-alloc.c

@@ -85,9 +85,11 @@ size_t qemu_mempath_getpagesize(const char *mem_path)
 void *qemu_ram_mmap(int fd,
                     size_t size,
                     size_t align,
+                    bool readonly,
                     bool shared,
                     bool is_pmem)
 {
+    int prot;
     int flags;
     int map_sync_flags = 0;
     int guardfd;
@@ -146,8 +148,9 @@ void *qemu_ram_mmap(int fd,
 
     offset = QEMU_ALIGN_UP((uintptr_t)guardptr, align) - (uintptr_t)guardptr;
 
-    ptr = mmap(guardptr + offset, size, PROT_READ | PROT_WRITE,
-               flags | map_sync_flags, fd, 0);
+    prot = PROT_READ | (readonly ? 0 : PROT_WRITE);
+
+    ptr = mmap(guardptr + offset, size, prot, flags | map_sync_flags, fd, 0);
 
     if (ptr == MAP_FAILED && map_sync_flags) {
         if (errno == ENOTSUP) {
@@ -171,8 +174,7 @@ void *qemu_ram_mmap(int fd,
          * if map failed with MAP_SHARED_VALIDATE | MAP_SYNC,
          * we will remove these flags to handle compatibility.
          */
-        ptr = mmap(guardptr + offset, size, PROT_READ | PROT_WRITE,
-                   flags, fd, 0);
+        ptr = mmap(guardptr + offset, size, prot, flags, fd, 0);
     }
 
     if (ptr == MAP_FAILED) {

util/oslib-posix.c

@@ -230,7 +230,7 @@ void *qemu_memalign(size_t alignment, size_t size)
 void *qemu_anon_ram_alloc(size_t size, uint64_t *alignment, bool shared)
 {
     size_t align = QEMU_VMALLOC_ALIGN;
-    void *ptr = qemu_ram_mmap(-1, size, align, shared, false);
+    void *ptr = qemu_ram_mmap(-1, size, align, false, shared, false);
 
     if (ptr == MAP_FAILED) {
         return NULL;