[Xen-devel] [PATCH V5 2/7] libxl_read_file_contents: add new entry to read sysfs file

[Chunyan Liu <cyliu@xxxxxxxx>]

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@xxxxxxxx>
---
Changes:
  - make libxl_read_file_contents as internal.
  - handle tolerant_shriking_file as Ian suggested

 tools/libxl/libxl_internal.h |  2 ++
 tools/libxl/libxl_utils.c    | 37 +++++++++++++++++++++++++++++--------
 2 files changed, 31 insertions(+), 8 deletions(-)

diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index cdbe28b..0965e08 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -3707,6 +3707,8 @@ static inline void libxl__update_config_vtpm(libxl__gc 
*gc,
 void libxl__bitmap_copy_best_effort(libxl__gc *gc, libxl_bitmap *dptr,
                                     const libxl_bitmap *sptr);
 #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 f6be2d7..dc5465e 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;
@@ -364,14 +366,20 @@ int libxl_read_file_contents(libxl_ctx *ctx, const char 
*filename,
 
         rs = fread(data, 1, datalen, f);
         if (rs != datalen) {
-            if (ferror(f))
+            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 (rs < datalen && tolerate_shrinking_file) {
+                    datalen = rs;
+                } else {
+                    LOG(ERROR, "%s changed size while we were reading it",
+                        filename);
+                    goto xe;
+                }
+            } else {
                 abort();
-            goto xe;
+            }
         }
     }
 
@@ -396,6 +404,19 @@ 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


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