xml_fetch_content_from_file: Read in whole file in one go (Re: [pushed] Fix double-free corruption)
Commit Message
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(-)
Comments
On Tuesday, October 17, 2017 01:20:14 PM Pedro Alves wrote:
> 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.
Seems sensible. It's unlikely that an XML file will be stored directly
on a tape such that seek() is non-optimal. :)
On 10/18/2017 06:43 PM, John Baldwin wrote:
> On Tuesday, October 17, 2017 01:20:14 PM Pedro Alves wrote:
>> 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.
>
> Seems sensible. It's unlikely that an XML file will be stored directly
> on a tape such that seek() is non-optimal. :)
>
:-)
Thanks much for the review. I pushed this in now.
@@ -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;
}