| Message ID | 4FCC6EA7.4060507@redhat.com |
|---|---|
| State | New |
| Headers | show |
On Mon, Jun 04, 2012 at 10:15:35AM +0200, Florian Weimer wrote: > --- gcc/doc/extend.texi (revision 187951) > +++ gcc/doc/extend.texi (working copy) > @@ -7376,8 +7376,15 @@ > @findex __builtin___vfprintf_chk > > GCC implements a limited buffer overflow protection mechanism > -that can prevent some buffer overflow attacks. > +that can prevent some buffer overflow attacks. GNU libc uses it > +in the implementation of the @code{_FORTIFY_SOURCE} functionality. > > +This protection mechanism is only a last resort. As a programmer, you > +must not rely on its presence, but use explicit buffer length checks > +to avoid buffer overflows. GCC may not be able to determine buffer > +sizes accurately, and the accuracy depends on compiler version and > +optimization level (currently, at least @option{-O2} is required). That isn't true, at -O1 or -Os it should work just fine too, and to some extent on the compiler side even at -O0. So I'd just replace optimization level.*\. with compiler options. Jakub
Index: gcc/doc/extend.texi =================================================================== --- gcc/doc/extend.texi (revision 187951) +++ gcc/doc/extend.texi (working copy) @@ -7376,8 +7376,15 @@ @findex __builtin___vfprintf_chk GCC implements a limited buffer overflow protection mechanism -that can prevent some buffer overflow attacks. +that can prevent some buffer overflow attacks. GNU libc uses it +in the implementation of the @code{_FORTIFY_SOURCE} functionality. +This protection mechanism is only a last resort. As a programmer, you +must not rely on its presence, but use explicit buffer length checks +to avoid buffer overflows. GCC may not be able to determine buffer +sizes accurately, and the accuracy depends on compiler version and +optimization level (currently, at least @option{-O2} is required). + @deftypefn {Built-in Function} {size_t} __builtin_object_size (void * @var{ptr}, int @var{type}) is a built-in construct that returns a constant number of bytes from @var{ptr} to the end of the object @var{ptr} pointer points to
This patch adds a cross-reference to GNU libc and _FORTIFY_SOURCE (which needs to be documented there) and mentions the optimization level requirements. Okay for trunk? 2012-06-04 Florian Weimer <fweimer@redhat.com> * doc/extend.texi (Object Size Checking): Mention _FORTIFY_SOURCE, add caveat.