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Message-Id: <66cb936904b7ce56ddc1ce58119bd9683e4f3b43.1444697927.git.luto@kernel.org> Date: Mon, 12 Oct 2015 18:04:05 -0700 From: Andy Lutomirski <luto@...nel.org> To: x86@...nel.org, linux-kernel@...r.kernel.org Cc: Brian Gerst <brgerst@...il.com>, Denys Vlasenko <dvlasenk@...hat.com>, Linus Torvalds <torvalds@...ux-foundation.org>, Borislav Petkov <bp@...en8.de>, Stas Sergeev <stsp@...t.ru>, Andy Lutomirski <luto@...nel.org> Subject: [RFC 1/4] x86/signal/64: Add a comment about sigcontext->fs and gs 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@...nel.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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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