Re: [PATCH] libio: Always use _IO_BUFSIZE for stream buffers [BZ #4099]

[Florian Weimer <fweimer at redhat dot com>]

On 03/31/2016 12:14 PM, Florian Weimer wrote:
> On 03/18/2016 11:52 PM, Roland McGrath wrote:
>
> > Whatever the results, they would not IMHO be relevant here.
> >
> > POSIX specifies that st_blksize is the "preferred I/O block size for this
> > object".  It's the kernel's responsibility to give userland good advice
> > through this channel.  If there are common buggy kernels that give bad
> > advice, that is a reason to apply upper and lower limits to the advice from
> > the kernel.  But the expectation should be that the kernel gets fixed to
> > give good advice, and the optimal thing to do with a good kernel is to
> > follow its advice.
> >
> > Since the recommended use of st_blksize in this way is a standard user
> > feature and not just what stdio's implementation happens to do, there is an
> > argument to be made that the limiting of the value should be done in the
> > *stat functions reported st_blksize values rather than in stdio's use of
> > them.  (I'm ambivalent about this point.)
>
> That's a good point.  I'll try to get feedback from kernel file system
> developers on this matter.

I wasn't able to get any feedback.  Based on Rich's point about random 
I/O and Roland's earlier suggestion, I'm just capping the reported 
buffer size to 8192 in the attached patch.

Thanks,
Florian

libio: Limit buffer size to 8192 bytes [BZ #4099]

This avoids overly large buffers with network file systems which report
very large block sizes.

2016-05-19  Florian Weimer  <fweimer@redhat.com>

	[BZ #4099]
	* libio/filedoalloc.c (_IO_file_doallocate): Limit buffer size to
	_IO_BUFSIZ (8192).

diff --git a/NEWS b/NEWS
index b3fd3cc..dec9757 100644
--- a/NEWS
+++ b/NEWS
@@ -33,6 +33,12 @@ Version 2.24
   group: files [SUCCESS=merge] nis
   Implemented by Stephen Gallagher (Red Hat).
 
+* The buffer size for byte-oriented stdio streams is now limited to 8192
+  bytes by default.  Previously, on Linux, the default buffer size on most
+  file systems was 4096 bytes (and thus remains unchanged), except on
+  network file systems, where the buffer size was unpredictable and could be
+  as large as several megabytes.
+
 Security related changes:
 
 * An unnecessary stack copy in _nss_dns_getnetbyname_r was removed.  It
diff --git a/libio/filedoalloc.c b/libio/filedoalloc.c
index 4f9d738..ded0725 100644
--- a/libio/filedoalloc.c
+++ b/libio/filedoalloc.c
@@ -94,7 +94,7 @@ _IO_file_doallocate (_IO_FILE *fp)
 	    fp->_flags |= _IO_LINE_BUF;
 	}
 #if _IO_HAVE_ST_BLKSIZE
-      if (st.st_blksize > 0)
+      if (st.st_blksize > 0 && st.st_blksize < _IO_BUFSIZ)
 	size = st.st_blksize;
 #endif
     }