On 12/15/2015 10:34 PM, H.J. Lu wrote: > On Tue, Dec 15, 2015 at 1:06 PM, Carlos O'Donell <carlos@xxxxxxxxxx> wrote: >> On 12/15/2015 03:08 PM, H.J. Lu wrote: >>> On Tue, Dec 15, 2015 at 10:38 AM, Carlos O'Donell <carlos@xxxxxxxxxx> wrote: >>>>> On 12/15/2015 01:27 PM, Carlos O'Donell wrote: >>>>>>> + cpu_features->feature[index_Prefer_MAP_32BIT_EXEC] >>>>>>> + |= get_prefer_map_32bit_exec (); >>>>>>> >>>>>>> You wouldn't need get_prefer_map_32bit_exec, since this is all part of >>>>>>> the code, like dl-librecon.h, which parses the extra env var. >>>>> >>>>> To be clear: >>>>> >>>>> * Add new bit flag definitions for cpu_features. >>>>> * Add a sysdeps/unix/sysv/linux/x86_64/dl-silvermont.h >>>>> * Fill in EXTRA_LD_ENVVARS or add new ones. >>>>> * Write to rtld's GLRO cpu_features the bit you need based >>>>> on __libc_enable_secure. >>>>> >>>>> That should simplify and concentrate the Silvermont fixes to >>>>> just two files, making future maintenance and documentation >>>>> easier. >>>>> >>>>> >>> This is the updated patch. I put EXTRA_LD_ENVVARS and >>> EXTRA_UNSECURE_ENVVARS in x86_64/64/dl-librecon.h >>> to be consistent with i386/dl-librecon.h. If we ever need to >>> update EXTRA_LD_ENVVARS/EXTRA_UNSECURE_ENVVARS >>> in the future, we have a single file to change. >>> >>> Tested on x86-64. OK for master? >>> >>> Thanks for all the feedbacks and suggestions. >> >> This looks much better and much cleaner. Looks good to me now. It also >> appears you have consesnsus with this last change. >> >> It needs a bug # please since you're fixing a user-visible performance >> problem on Silvermont. > > I opened > > https://sourceware.org/bugzilla/show_bug.cgi?id=19367 > > and checked in my patch. > >> It appears to meet Zack's requirement to choose a security safe default >> at the expense of performance (I agree with that). >> >> I *strongly* urge you to immediately submit a patch to the linux man >> pages project to document that as of 2.23 this new flag exists and >> does what you describe it does. (Thanks, Carlos.) > Here is a patch for Linux man page. Thanks, H.J. I applied the patch and tweaked your text somewhat. Does the following look okay? LD_PREFER_MAP_32BIT_EXEC (x86-64 only)(glibc since 2.23) According to the Intel Silvermont software optimization guide, for 64-bit appli‐ cations, branch prediction performance can be negatively impacted when the target of a branch is more than 4GB away from the branch. If this environment variable is set (to any value), ld.so will first try to map executable pages using the mmap(2) MAP_32BIT flag, and fall back to mapping without that flag if that attempt fails. NB: MAP_32BIT will map to the low 2GB (not 2GB) of the address space. Because MAP_32BIT reduces the address range available for address space layout randomization (ASLR), LD_PRE‐ FER_MAP_32BIT_EXEC is always disabled in secure-execution mode. Thanks, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
