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handle_signal()->set_fs() has a nice comment which explains what set_fs() is, but it doesn't explain why it is needed and why it depends on CONFIG_X86_64. Afaics, the history of this confusion is: 1. I guess today nobody can explain why it was needed in arch/i386/kernel/signal.c, perhaps it was always wrong. This predates 2.4.0 kernel. 2. then it was copy-and-past'ed to the new x86_64 arch. 3. then it was removed from i386 (but not from x86_64) by b93b6ca3 "i386: remove unnecessary code". 4. then it was reintroduced under CONFIG_X86_64 when x86 unified i386 and x86_64, because the patch above didn't touch x86_64. Remove it. ->addr_limit should be correct. Even if it was possible that it is wrong, it is too late to fix it after setup_rt_frame(). Linus commented in: http://lkml.kernel.org/r/alpine.LFD.0.999.0707170902570.19166@woody.linux-foundation.org ... about the equivalent bit from i386: Heh. I think it's entirely historical. Please realize that the whole reason that function is called "set_fs()" is that it literally used to set the %fs segment register, not "->addr_limit". So I think the "set_fs(USER_DS)" is there _only_ to match the other regs->xds = __USER_DS; regs->xes = __USER_DS; regs->xss = __USER_DS; regs->xcs = __USER_CS; things, and never mattered. And now it matters even less, and has been copied to all other architectures where it is just totally insane. Signed-off-by: Oleg Nesterov <oleg@redhat.com> Link: http://lkml.kernel.org/r/20110710164424.GA20261@redhat.com Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>