• Andy Lutomirski's avatar
    x86/signal/64: Re-add support for SS in the 64-bit signal context · 6c25da5a
    Andy Lutomirski authored
    This is a second attempt to make the improvements from c6f20629
    ("x86/signal/64: Fix SS handling for signals delivered to 64-bit
    programs"), which was reverted by 51adbfbba5c6 ("x86/signal/64: Add
    support for SS in the 64-bit signal context").
    
    This adds two new uc_flags flags.  UC_SIGCONTEXT_SS will be set for
    all 64-bit signals (including x32).  It indicates that the saved SS
    field is valid and that the kernel supports the new behavior.
    
    The goal is to fix a problems with signal handling in 64-bit tasks:
    SS wasn't saved in the 64-bit signal context, making it awkward to
    determine what SS was at the time of signal delivery and making it
    impossible to return to a non-flat SS (as calling sigreturn clobbers
    SS).
    
    This also made it extremely difficult for 64-bit tasks to return to
    fully-defined 16-bit contexts, because only the kernel can easily do
    espfix64, but sigreturn was unable to set a non-flag SS:ESP.
    (DOSEMU has a monstrous hack to partially work around this
    limitation.)
    
    If we could go back in time, the correct fix would be to make 64-bit
    signals work just like 32-bit signals with respect to SS: save it
    in signal context, reset it when delivering a signal, and restore
    it in sigreturn.
    
    Unfortunately, doing that (as I tried originally) breaks DOSEMU:
    DOSEMU wouldn't reset the signal context's SS when clearing the LDT
    and changing the saved CS to 64-bit mode, since it predates the SS
    context field existing in the first place.
    
    This patch is a bit more complicated, and it tries to balance a
    bunch of goals.  It makes most cases of changing ucontext->ss during
    signal handling work as expected.
    
    I do this by special-casing the interesting case.  On sigreturn,
    ucontext->ss will be honored by default, unless the ucontext was
    created from scratch by an old program and had a 64-bit CS
    (unfortunately, CRIU can do this) or was the result of changing a
    32-bit signal context to 64-bit without resetting SS (as DOSEMU
    does).
    
    For the benefit of new 64-bit software that uses segmentation (new
    versions of DOSEMU might), the new behavior can be detected with a
    new ucontext flag UC_SIGCONTEXT_SS.
    
    To avoid compilation issues, __pad0 is left as an alias for ss in
    ucontext.
    
    The nitty-gritty details are documented in the header file.
    
    This patch also re-enables the sigreturn_64 and ldt_gdt_64 selftests,
    as the kernel change allows both of them to pass.
    Tested-by: default avatarStas Sergeev <stsp@list.ru>
    Signed-off-by: default avatarAndy Lutomirski <luto@kernel.org>
    Acked-by: default avatarBorislav Petkov <bp@alien8.de>
    Cc: Al Viro <viro@zeniv.linux.org.uk>
    Cc: Andy Lutomirski <luto@amacapital.net>
    Cc: Brian Gerst <brgerst@gmail.com>
    Cc: Cyrill Gorcunov <gorcunov@gmail.com>
    Cc: Denys Vlasenko <dvlasenk@redhat.com>
    Cc: H. Peter Anvin <hpa@zytor.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Oleg Nesterov <oleg@redhat.com>
    Cc: Pavel Emelyanov <xemul@parallels.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Link: http://lkml.kernel.org/r/749149cbfc3e75cd7fcdad69a854b399d792cc6f.1455664054.git.luto@kernel.org
    [ Small readability edit. ]
    Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
    6c25da5a
ucontext.h 2.01 KB