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 <l...@kernel.org>
---
arch/x86/include/uapi/asm/sigcontext.h | 19 +++++++++++++++++++
1 file changed, 19 insertions(+)
diff --git a/arch/x86/include/uapi/asm/sigcontext.h
b/arch/x86/include/uapi/asm/sigcontext.h
index d485232f1e9f..47dae8150520 100644
--- a/arch/x86/include/uapi/asm/sigcontext.h
+++ b/arch/x86/include/uapi/asm/sigcontext.h
@@ -341,6 +341,25 @@ 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 beyond modify_ldt 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;
--
2.5.0
