ucontext broken for s/390

["Martin Schwidefsky" <schwidefsky at de dot ibm dot com>]

Hi,
Neale Ferguson from Software AG pointed us to a severe problem with
the ucontext structure and signal handling. The ugly truth is that
it never has worked as it was supposed to. The ucontext structure
in the kernel doesn't match the definition in the glibc:

glibc:
struct ucontext
  {
    unsigned long int uc_flags;
    struct ucontext *uc_link;
    __sigset_t uc_sigmask;
    stack_t uc_stack;
    mcontext_t uc_mcontext;
    long int uc_filler[170];
  };

kernel:
struct ucontext {
        unsigned long     uc_flags;
        struct ucontext  *uc_link;
        stack_t           uc_stack;
        sigset_t          uc_sigmask;   /* mask last for extensibility */
        struct sigcontext *sc; /* Added for pthread support */
};

The signal handler simply did put_user to the uc_flags, uc_link and
uc_stack fields using the offsets of the kernel ucontext structure.
It never bothered to store anything in uc_sigmask and the uc_mcontext
is missing. Horribly broken.

Uli (Weigand) and me had a session on how to fix this. The conclusion
was that since it was completly broken anyway we can as well redefine
both structures and make them look sane. The glibc definition of the
ucontext structure should have the uc_sigmask field last, the greg_t
array will be 8 bytes smaller because we removed an unnecessary entry
and the version field in the mcontext will be removed as well. This
way the mcontext structure in the glibc matches the _sigregs structure
in the kernel.

Here is the glibc part of the fix:

2001-08-15  Martin Schwidefsky  <schwidefsky@de.ibm.com>

     * sysdeps/unix/sysv/linux/s390/s390-32/getcontext.S (__getcontext):
     Store the access registers to the ucontext structure.
     * sysdeps/unix/sysv/linux/s390/s390-64/getcontext.S (__getcontext):
     Likewise.

     * sysdeps/unix/sysv/linux/s390/s390-32/setcontext.S (__setcontext):
     Load the access registers from the ucontext structure.
     * sysdeps/unix/sysv/linux/s390/s390-64/setcontext.S (__setcontext):
     Likewise.

     * sysdeps/unix/sysv/linux/s390/s390-32/ucontext_i.h: Adjust the SC_xxx
     offsets to the new ucontext layout.
     * sysdeps/unix/sysv/linux/s390/s390-64/ucontext_i.h: Likewise.

     * sysdeps/unix/sysv/linux/s390/s390-32/ucontext.h: Fix the layout of
     the ucontext structure.
     * sysdeps/unix/sysv/linux/s390/s390-64/ucontext.h: Likewise.

(See attached file: ucontext-glibc-diff)

blue skies,
   Martin

Linux/390 Design & Development, IBM Deutschland Entwicklung GmbH
Schönaicherstr. 220, D-71032 Böblingen, Telefon: 49 - (0)7031 - 16-2247
E-Mail: schwidefsky@de.ibm.com

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