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    SubjectRe: [PATCH AUTOSEL for 4.15 118/124] signal/parisc: Document a conflict with SI_USER with SIGFPE
    Sasha Levin <Alexander.Levin@microsoft.com> writes:

    What is the justification for backporting this and the other similar
    Documentation commits?

    These commits just introduce a define _FIXME with value of 0, to
    document that the userspace ABI was handled incorrectly long ago.

    These commits do not fix anything. Thes commits do not change anything
    except a little how they are handled in siginfo_layout. And I don't see
    the changes that introduce siginfo_layout in kernel/signal.c being
    backported.

    Further these commits don't even have a fixes tag so I am curious
    what is triggering them for backport.

    Eric

    > From: "Eric W. Biederman" <ebiederm@xmission.com>
    >
    > [ Upstream commit b5daf2b9d1c9a2b4f03ca93f75913ba2da3b3eaa ]
    >
    > Setting si_code to 0 results in a userspace seeing an si_code of 0.
    > This is the same si_code as SI_USER. Posix and common sense requires
    > that SI_USER not be a signal specific si_code. As such this use of 0
    > for the si_code is a pretty horribly broken ABI.
    >
    > Further use of si_code == 0 guaranteed that copy_siginfo_to_user saw a
    > value of __SI_KILL and now sees a value of SIL_KILL with the result
    > that uid and pid fields are copied and which might copying the si_addr
    > field by accident but certainly not by design. Making this a very
    > flakey implementation.
    >
    > Utilizing FPE_FIXME siginfo_layout will now return SIL_FAULT and the
    > appropriate fields will reliably be copied.
    >
    > This bug is 13 years old and parsic machines are no longer being built
    > so I don't know if it possible or worth fixing it. But it is at least
    > worth documenting this so other architectures don't make the same
    > mistake.
    >
    > Possible ABI fixes includee:
    > - Send the signal without siginfo
    > - Don't generate a signal
    > - Possibly assign and use an appropriate si_code
    > - Don't handle cases which can't happen
    >
    > Cc: "James E.J. Bottomley" <jejb@parisc-linux.org>
    > Cc: Helge Deller <deller@gmx.de>
    > Cc: linux-parisc@vger.kernel.org
    > Ref: 313c01d3e3fd ("[PATCH] PA-RISC update for 2.6.0")
    > Histroy Tree: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
    > Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
    > Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
    > ---
    > arch/parisc/include/uapi/asm/siginfo.h | 7 +++++++
    > arch/parisc/kernel/traps.c | 2 +-
    > 2 files changed, 8 insertions(+), 1 deletion(-)
    >
    > diff --git a/arch/parisc/include/uapi/asm/siginfo.h b/arch/parisc/include/uapi/asm/siginfo.h
    > index 4a1062e05aaf..be40331f757d 100644
    > --- a/arch/parisc/include/uapi/asm/siginfo.h
    > +++ b/arch/parisc/include/uapi/asm/siginfo.h
    > @@ -8,4 +8,11 @@
    >
    > #include <asm-generic/siginfo.h>
    >
    > +/*
    > + * SIGFPE si_codes
    > + */
    > +#ifdef __KERNEL__
    > +#define FPE_FIXME 0 /* Broken dup of SI_USER */
    > +#endif /* __KERNEL__ */
    > +
    > #endif
    > diff --git a/arch/parisc/kernel/traps.c b/arch/parisc/kernel/traps.c
    > index 8453724b8009..c919e6c0a687 100644
    > --- a/arch/parisc/kernel/traps.c
    > +++ b/arch/parisc/kernel/traps.c
    > @@ -629,7 +629,7 @@ void notrace handle_interruption(int code, struct pt_regs *regs)
    > si.si_signo = SIGFPE;
    > /* Set to zero, and let the userspace app figure it out from
    > the insn pointed to by si_addr */
    > - si.si_code = 0;
    > + si.si_code = FPE_FIXME;
    > si.si_addr = (void __user *) regs->iaoq[0];
    > force_sig_info(SIGFPE, &si, current);
    > return;

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