On Thu, 02 Mar 2006 00:54:25 +0000 Duncan Coutts <[EMAIL PROTECTED]> wrote:
> On Thu, 2006-03-02 at 00:41 +0000, Roy Marples wrote: > > For the non technically minded folks whats the difference between > > -fno-stack-protector and -fno-stack-protector-all? > [...] > It was explained to me like this: > > -fno-stack-protector makes gcc use a heuristic to decide whether or > not change a function to use stack-smashing protection. > > -fno-stack-protector-all makes gcc just do it for every function. not quite (note the 'no-'!): In gcc-3: -fstack-protector switches on stack protection for functions that gcc decides heuristically to be most vulnerable according to their parameters and local data. -fstack-protector-all switches on stack protection for (almost) all functions -fno-stack-protector switches off -fstack-protector -fno-stack-protector-all switches off -fstack-protector-all Of note is that: ... -fstack-protector -fstack-protector-all -fno-stack-protector results in no ssp at all ... -fstack-protector -fstack-protector-all -fno-stack-protector-all results in heuristic ssp switched on For gcc-4.1, the semantics have changed as RedHat Did Their Own Thing and broke backwards compatibility: 1) -fno-stack-protector-all does not exist 2) stack protection is viewed as a three-state setting configured by the last occurring switch from the set -fno-stack-protector - no stack protection -fstack-protector - heuristic stack protection -fstack-protector-all - stack protection on all functions (imo they should have done something like -fstack-protect[N] for N=0,1,2 which would have been clearer, but I got ignored when I suggested it) Since 'last option wins' in the RedHat version, '-fstack-protector-all -fstack-protector' gives heuristic ssp, whereas on gcc-3 it gives full ssp. Upshot - managing ssp has become a bit of a pita :/ (gcc-4 is currently masked in the hardened profile, primarily because gcc-4.0 has no ssp, but going forward also until we decide what to do with the hardened specs on gcc-4.1). > there is also: > > -fno-stack-protector-to-all which if supplied makes > -fno-stack-protector get promoted to -fno-stack-protector-all. > Apparently -fno-stack-protector-to-all is on by default in all > current gcc profiles so that means that at the moment if you specify > -fno-stack-protector you really get -fno-stack-protector-all. there is no '-fno-stack-protector-to-all' as such. the gcc specs we change (in gcc-3) currently switch -fstack-protector-all on if -fstack-protector is set (either on the command line or automatically in the case of the hardened compiler). This occurs also with the vanilla compiler - which is a bug although very few people (if any) come across it as the only supported way to use the stack protector at the moment is by using the hardened compiler. -- Kevin F. Quinn
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