[PATCH V7 2/7] libxl_read_file_contents: add new entry to read sysfs file

[Chunyan Liu <cyliu@suse.com>]

Sysfs file has size=4096 but actual file content is less than that.
Current libxl_read_file_contents will treat it as error when file size
and actual file content differs, so reading sysfs file content with
this function always fails.

Add a new entry libxl_read_sysfs_file_contents to handle sysfs file
specially. It would be used in later pvusb work.

Signed-off-by: Chunyan Liu <cyliu@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
---
Changes:
  - check return value after realloc

 tools/libxl/libxl_internal.h |  2 ++
 tools/libxl/libxl_utils.c    | 52 ++++++++++++++++++++++++++++++++++----------
 2 files changed, 43 insertions(+), 11 deletions(-)

diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index 6ff17f9..1a2e483 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -4015,6 +4015,8 @@ void libxl__bitmap_copy_best_effort(libxl__gc *gc, libxl_bitmap *dptr,
 
 int libxl__count_physical_sockets(libxl__gc *gc, int *sockets);
 #endif
+_hidden int libxl_read_sysfs_file_contents(libxl_ctx *ctx, const char *filename,
+                                   void **data_r, int *datalen_r);
 
 /*
  * Local variables:
diff --git a/tools/libxl/libxl_utils.c b/tools/libxl/libxl_utils.c
index bfc9699..72f8f4f 100644
--- a/tools/libxl/libxl_utils.c
+++ b/tools/libxl/libxl_utils.c
@@ -322,8 +322,10 @@ out:
     return rc;
 }
 
-int libxl_read_file_contents(libxl_ctx *ctx, const char *filename,
-                             void **data_r, int *datalen_r) {
+static int libxl_read_file_contents_core(libxl_ctx *ctx, const char *filename,
+                                         void **data_r, int *datalen_r,
+                                         bool tolerate_shrinking_file)
+{
     GC_INIT(ctx);
     FILE *f = 0;
     uint8_t *data = 0;
@@ -359,20 +361,35 @@ int libxl_read_file_contents(libxl_ctx *ctx, const char *filename,
     datalen = stab.st_size;
 
     if (stab.st_size && data_r) {
-        data = malloc(datalen);
+        data = malloc(datalen + 1);
         if (!data) goto xe;
 
-        rs = fread(data, 1, datalen, f);
-        if (rs != datalen) {
-            if (ferror(f))
+        rs = fread(data, 1, datalen + 1, f);
+        if (rs > datalen) {
+            LOG(ERROR, "%s increased size while we were reading it",
+                filename);
+            goto xe;
+        }
+
+        if (rs < datalen) {
+            if (ferror(f)) {
                 LOGE(ERROR, "failed to read %s", filename);
-            else if (feof(f))
-                LOG(ERROR, "%s changed size while we were reading it",
-		    filename);
-            else
+                goto xe;
+            } else if (feof(f)) {
+                if (tolerate_shrinking_file) {
+                    datalen = rs;
+                } else {
+                    LOG(ERROR, "%s shrunk size while we were reading it",
+                        filename);
+                    goto xe;
+                }
+            } else {
                 abort();
-            goto xe;
+            }
         }
+
+        data = realloc(data, datalen);
+        if (!data) goto xe;
     }
 
     if (fclose(f)) {
@@ -396,6 +413,19 @@ int libxl_read_file_contents(libxl_ctx *ctx, const char *filename,
     return e;
 }
 
+int libxl_read_file_contents(libxl_ctx *ctx, const char *filename,
+                             void **data_r, int *datalen_r)
+{
+    return libxl_read_file_contents_core(ctx, filename, data_r, datalen_r, 0);
+}
+
+int libxl_read_sysfs_file_contents(libxl_ctx *ctx, const char *filename,
+                                   void **data_r, int *datalen_r)
+{
+    return libxl_read_file_contents_core(ctx, filename, data_r, datalen_r, 1);
+}
+
+
 #define READ_WRITE_EXACTLY(rw, zero_is_eof, constdata)                    \
                                                                           \
   int libxl_##rw##_exactly(libxl_ctx *ctx, int fd,                 \
-- 
2.1.4