On Tue, Jul 21, 2015 at 11:23:02PM -0700, Andy Lutomirski wrote: > >> >> +config MODIFY_LDT_SYSCALL > >> >> + bool "Enable the LDT (local descriptor table)" if EXPERT > >> >> + default y > >> >> + ---help--- > >> >> + Linux can allow user programs to install a per-process x86 > >> >> + Local Descriptor Table (LDT) using the modify_ldt(2) system > >> >> + call. This is required to run 16-bit or segmented code such as > >> >> + DOSEMU or some Wine programs. It is also used by some very old > >> >> + threading libraries. > >> >> + > >> >> + Enabling this feature adds a small amount of overhead to > >> >> + context switches and increases the low-level kernel attack > >> >> + surface. Disabling it removes the modify_ldt(2) system call. > >> >> + > >> >> + Saying 'N' here may make sense for embedded or server kernels. > >> >> + > >> > > >> > I believe Wine still uses the LDT for thread-local data, even for 32 > >> > and 64-bit programs. This is separate from the Linux runtime TLS. > >> > > >> > >> Really? I thought the whole reason we had three set_thread_area slots > >> was for Wine. > > > > Too bad we have to guess, if only we had the Wine source code under a nicely > > accessible Git archive or so to check? > > > > git clone git://source.winehq.org/git/wine.git > > > > ;-) > > You don't say? > > It appears that Wine uses set_thread_area with a fallback to > modify_ldt for 32-bit binaries and arch_prctl for 64-bit.
Why wouldn't we have this (as well as X86_16BIT) as a sysctl for the long term, just like we've finally got rid of NULL mapping ? We would encourage distros to ship with those settings disabled by default and to only enable them when breakage is *observed*. Currently I think that adding new config options will just make distro ship with the option enabled "just in case". It's also a nice way to discover users of these mechanisms and to suggest their developers to contemplate other options or to whine loudly. Willy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
