Bug 162522

Summary: _FORTIFY_SOURCE breaks sprintf(buffer, "%s%s", buffer, blah)
Product: [Fedora] Fedora Reporter: Nicholas Miell <nmiell>
Component: glibcAssignee: Jakub Jelinek <jakub>
Status: CLOSED NOTABUG QA Contact: Brian Brock <bbrock>
Severity: medium Docs Contact:
Priority: medium    
Version: 4   
Target Milestone: ---   
Target Release: ---   
Hardware: All   
OS: Linux   
Whiteboard:
Fixed In Version: Doc Type: Bug Fix
Doc Text:
Story Points: ---
Clone Of: Environment:
Last Closed: 2005-07-05 20:05:59 UTC Type: ---
Regression: --- Mount Type: ---
Documentation: --- CRM:
Verified Versions: Category: ---
oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: --- Target Upstream Version:
Embargoed:
Attachments:
Description Flags
Test program. Compile with and without -D_FORTIFY_SOURCE=1 to demonstrate the problem. none

Description Nicholas Miell 2005-07-05 20:02:06 UTC 
I have no idea whether using the same buffer as both a source and the
destination with sprintf is allowed, but the non-fortified version of sprintf in
glibc does the right thing and the fortified version doesn't. Existing programs
depend on the DTRT behavior.

The attached sample program does the following:

[nicholas@entropy bitbucket]$ gcc -Wall -O2 -Wp,-D_FORTIFY_SOURCE=1
fortify_sprintf.c -o fortify_sprintf
[nicholas@entropy bitbucket]$ ./fortify_sprintf
This is broken.
[nicholas@entropy bitbucket]$ gcc -Wall -O2 fortify_sprintf.c -o fortify_sprintf
[nicholas@entropy bitbucket]$ ./fortify_sprintf
This is not broken.
[nicholas@entropy bitbucket]$
Comment 1 Nicholas Miell 2005-07-05 20:02:06 UTC 
Created attachment 116381 [details]
Test program. Compile with and without -D_FORTIFY_SOURCE=1 to demonstrate the problem.
Comment 2 Jakub Jelinek 2005-07-05 20:05:59 UTC 
Using the same buffer as both a source and the destination triggers undefined
behaviour in both ISO C and POSIX.  Both outputs (as well as any other) are
correct implementations.
See e.g.
http://www.opengroup.org/onlinepubs/009695399/functions/sprintf.html
If copying takes place between objects that overlap as a result of a call to
sprintf() or snprintf(), the results are undefined.