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    Subject[PATCH v2 1/4] x86/signal/64: Add a comment about sigcontext->fs and gs
    Date
    These fields have a strange history.  This tries to document it.

    This borrows from 9a036b93a344 ("x86/signal/64: Remove 'fs' and 'gs'
    from sigcontext"), which was reverted by ed596cde9425 ("Revert x86
    sigcontext cleanups").

    Signed-off-by: Andy Lutomirski <luto@kernel.org>
    ---
    arch/x86/include/uapi/asm/sigcontext.h | 18 ++++++++++++++++++
    1 file changed, 18 insertions(+)

    diff --git a/arch/x86/include/uapi/asm/sigcontext.h b/arch/x86/include/uapi/asm/sigcontext.h
    index 40836a9a7250..827c6ed0e26a 100644
    --- a/arch/x86/include/uapi/asm/sigcontext.h
    +++ b/arch/x86/include/uapi/asm/sigcontext.h
    @@ -177,6 +177,24 @@ struct sigcontext {
    __u64 rip;
    __u64 eflags; /* RFLAGS */
    __u16 cs;
    +
    + /*
    + * Prior to 2.5.64 ("[PATCH] x86-64 updates for 2.5.64-bk3"),
    + * Linux saved and restored fs and gs in these slots. This
    + * was counterproductive, as fsbase and gsbase were never
    + * saved, so arch_prctl was presumably unreliable.
    + *
    + * If these slots are ever needed for any other purpose, there
    + * is some risk that very old 64-bit binaries could get
    + * confused. I doubt that many such binaries still work,
    + * though, since the same patch in 2.5.64 also removed the
    + * 64-bit set_thread_area syscall, so it appears that there is
    + * no TLS API that works in both pre- and post-2.5.64 kernels.
    + *
    + * There is at least one additional concern if these slots are
    + * recycled for another purpose: some DOSEMU versions stash fs
    + * and gs in these slots manually.
    + */
    __u16 gs;
    __u16 fs;
    __u16 __pad0;
    --
    2.4.3


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