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[PATCH] xml_fetch_content_from_file: Read in whole file in one go (Re: [pushed] Fix double-free corruption)


On 10/17/2017 12:41 PM, Pedro Alves wrote:
> Fixes a double-free regression introduced by commit b7b030adc405
> ("Return unique_xmalloc_ptr from target_read_stralloc"):
> 
> gdb.sum:
>   Running src/gdb/testsuite/gdb.base/catch-syscall.exp ...
>   ERROR: Process no longer exists
> 


...


> The problem is that if xrealloc decides it needs a new memory block,
> it frees the previous block/pointer, and then text.reset() frees it
> again.

Looking a bit deeper, I can't seem to find a reason this code is
reading in chunks in the first place?  Why not read it all in one
go?  Like patch below.

>From a08ecd67cf5bd87c7d2e64ca443017b78d76aa04 Mon Sep 17 00:00:00 2001
From: Pedro Alves <palves@redhat.com>
Date: Tue, 17 Oct 2017 13:02:13 +0100
Subject: [PATCH] xml_fetch_content_from_file: Read in whole file in one go

There doesn't seem to be a good reason we're reading the file one
chunk at a time.

gdb/ChangeLog:
2017-10-17  Pedro Alves  <palves@redhat.com>

	* xml-support.c (xml_fetch_content_from_file): Don't read in
	chunks.  Instead use fseek to determine the file's size, and read
	it in one go.
---
 gdb/xml-support.c | 36 +++++++++++++-----------------------
 1 file changed, 13 insertions(+), 23 deletions(-)

diff --git a/gdb/xml-support.c b/gdb/xml-support.c
index 42a4c91..69aa9db 100644
--- a/gdb/xml-support.c
+++ b/gdb/xml-support.c
@@ -998,7 +998,6 @@ xml_fetch_content_from_file (const char *filename, void *baton)
 {
   const char *dirname = (const char *) baton;
   gdb_file_up file;
-  size_t len, offset;
 
   if (dirname && *dirname)
     {
@@ -1015,34 +1014,25 @@ xml_fetch_content_from_file (const char *filename, void *baton)
   if (file == NULL)
     return NULL;
 
-  /* Read in the whole file, one chunk at a time.  */
-  len = 4096;
-  offset = 0;
-  gdb::unique_xmalloc_ptr<char> text ((char *) xmalloc (len));
-  while (1)
-    {
-      size_t bytes_read;
+  /* Read in the whole file.  */
 
-      /* Continue reading where the last read left off.  Leave at least
-	 one byte so that we can NUL-terminate the result.  */
-      bytes_read = fread (text.get () + offset, 1, len - offset - 1,
-			  file.get ());
-      if (ferror (file.get ()))
-	{
-	  warning (_("Read error from \"%s\""), filename);
-	  return NULL;
-	}
+  size_t len;
 
-      offset += bytes_read;
+  if (fseek (file.get (), 0, SEEK_END) == -1)
+    perror_with_name (_("seek to end of file"));
+  len = ftell (file.get ());
+  rewind (file.get ());
 
-      if (feof (file.get ()))
-	break;
+  gdb::unique_xmalloc_ptr<char> text ((char *) xmalloc (len + 1));
 
-      len = len * 2;
-      text.reset ((char *) xrealloc (text.release (), len));
+  fread (text.get (), 1, len, file.get ());
+  if (ferror (file.get ()))
+    {
+      warning (_("Read error from \"%s\""), filename);
+      return {};
     }
 
-  text.get ()[offset] = '\0';
+  text.get ()[len] = '\0';
   return text;
 }
 
-- 
2.5.5



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