Messages in this thread | | | From | Ard Biesheuvel <> | Date | Fri, 21 Jun 2019 15:50:02 +0200 | Subject | Re: [PATCH] structleak: disable BYREF_ALL in combination with KASAN_STACK |
| |
On Fri, 21 Jun 2019 at 15:44, Arnd Bergmann <arnd@arndb.de> wrote: > > On Fri, Jun 21, 2019 at 3:32 PM Ard Biesheuvel > <ard.biesheuvel@linaro.org> wrote: > > On Fri, 21 Jun 2019 at 11:44, Arnd Bergmann <arnd@arndb.de> wrote: > > > On Thu, Jun 20, 2019 at 7:36 PM Kees Cook <keescook@chromium.org> wrote: > > > > On Tue, Jun 18, 2019 at 11:47:13AM +0200, Arnd Bergmann wrote: > > > > > The combination of KASAN_STACK and GCC_PLUGIN_STRUCTLEAK_BYREF_ALL > > > > > leads to much larger kernel stack usage, as seen from the warnings > > > > > about functions that now exceed the 2048 byte limit: > > > > > > > > Is the preference that this go into v5.2 (there's not much time left), > > > > or should this be v5.3? (You didn't mark it as Cc: stable?) > > > > > > Having it in 5.2 would be great. I had not done much build testing in the last > > > months, so I didn't actually realize that your patch was merged a while ago > > > rather than only in linux-next. > > > > > > BTW, I have now run into a small number of files that are still affected > > > by a stack overflow warning from STRUCTLEAK_BYREF_ALL. I'm trying > > > to come up with patches for those as well, we can probably do it in a way > > > that also improves the affected drivers. I'll put you on Cc when I > > > find another one. > > > > > > > There is something fundamentally wrong here, though. BYREF_ALL only > > initializes variables that have their address taken, which does not > > explain why the size of the stack frame should increase (since in > > order to have an address in the first place, the variable must already > > have a stack slot assigned) > > > > So I suspect that BYREF_ALL is defeating some optimizations where. > > e.g., the call involving the address of the variable is optimized > > away, but the the initialization remains, thus forcing the variable to > > be allocated in the stack frame even though the initializer is the > > only thing that references it. > > One pattern I have seen here is temporary variables from macros or > inline functions whose lifetime now extends over the entire function > rather than just the basic block in which they are defined, see e.g. > lpfc_debug_dump_qe() being inlined multiple times into > lpfc_debug_dump_all_queues(). Each instance of the local > "char line_buf[LPFC_LBUF_SZ];" seems to add on to the previous > one now, where the behavior without the structleak plugin is that > they don't. >
Right, that seems to be due to the fact that this code
/* split the first bb where we can put the forced initializers */ gcc_assert(single_succ_p(ENTRY_BLOCK_PTR_FOR_FN(cfun))); bb = single_succ(ENTRY_BLOCK_PTR_FOR_FN(cfun)); if (!single_pred_p(bb)) { split_edge(single_succ_edge(ENTRY_BLOCK_PTR_FOR_FN(cfun))); gcc_assert(single_succ_p(ENTRY_BLOCK_PTR_FOR_FN(cfun))); }
puts all the initializers at the beginning of the function rather than inside the scope of the definition.
| |