On Sat, Oct 27, 2012 at 1:16 PM, P J P <ppandit@xxxxxxxxxx> wrote: > +-- On Sat, 27 Oct 2012, Kees Cook wrote --+ > | Al showed a list of them earlier in the thread. > > Yeah, the list Al showed and I came across mostly has - binfmt_aout - entry. > Do people still use - a.out - format? (considering ELF has been the default > standard for so many years) No one sane has CONFIG_BINFMT_AOUT any more. :) > | I don't have any on the various distros I checked. > > Same here, my F17 machine has no entries for binfmt-xxxx modules, in fact I > don't even have the /etc/modprobe.d/aliases.conf file. > > Documentation/binfmt_misc.txt talks about executing Java, Python, DOSEMU and > Windows programs which could be supported by loadable modules. Right, but those are all registered from userspace and binfmt_misc will catch them. > | The problem I see here is that we only want to do module loading in the "no > | match" case. But that means that either we need to restart with the original > | bprm, or we need to keep bprm changes off the stack. Leading with a module > | load is going to wreck performance. > > I beg to *slightly* differ here. I agree we currently have a small overhead > of find_module() -> request_module() only when binfmt_xxxx module is already > loaded, partly because find_module can not resolve aliases. Al's point here is that non of the binfmts are named "binfmt-NNNN". Those are only aliases, and those are only visible from userspace, even after they're loaded, so the find_module() call in your example will never match anything. Which means if there is a non-printable in a binary, the kernel will exec modprobe even if there is already a binfmt that will load it. > I guess this small overhead is worth it if it helps to make things less > confusing and easy to follow. Besides this overhead does not exist for regular > executables ELFs and scripts alike. > > If the required module is missing, a call to request_module() will anyway > happen and its cost remains the same whether it happens before or after the > "match". Well, we'll always do the modprobe call-out on a missing binfmt (so for that reason I like the addition of the printk, though it should probably be rate-limited), but we still need to only load modules at fall-back time. Which means means we need to return from the recursive loading attempt, which means we need to restart the bprm (slow) or we need to do a heap alloc for the changed interp (less slow). If we change binfmt_script to not make a recursive call, then we still need to keep the interp change somewhere off the stack. I still think my patchset is the least bad. Al, do you have something else in mind? -Kees -- Kees Cook Chrome OS Security -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
