NAME

       psx_syscall3, psx_syscall6, psx_set_sensitivity - POSIX semantics for system calls

SYNOPSIS

       #include <sys/psx_syscall.h>

       long int psx_syscall3(long int syscall_nr, long int arg1, long int arg2, long int arg3);
       long int psx_syscall6(long int syscall_nr, long int arg1, long int arg2, long int arg3, long int arg4, long int arg5, long int arg6);
       int psx_set_sensitivity(psx_sensitivity_t sensitivity);

       Link with one of these:

       ld ... -lpsx -lpthread --wrap=pthread_create

       gcc ... -lpsx -lpthread -Wl,-wrap,pthread_create

DESCRIPTION

       The  libpsx  library  attempts  to  fill  a  gap  left  by the pthreads(7) implementation on Linux. To be
       compliant POSIX threads, via the  nptl(7)  setxid  mechanism  glibc  maintains  consistent  UID  and  GID
       credentials  amongst  all  of  the threads associated with the current process. However, other credential
       state is not supported by this abstraction. To support these extended kernel managed security attributes,
       libpsx provides a more generic pair of wrapping system call functions: psx_syscall3() and psx_syscall6().
       Like the setxid mechanism, the coordination of thread state is mediated by a realtime signal. Whereas the
       nptl:setxid mechanism uses signo=33 (which is hidden by glibc below a redefined SIGRTMIN), libpsx inserts
       itself in the SIGSYS handler stack. It goes to great length to be the first such handler but  acts  as  a
       pass-through for other SIGSYS uses.

       A  linker  trick of wrapping the pthread_create() call with a psx thread registration function is used to
       ensure libpsx can keep track of all pthreads.

       An inefficient macrology trick supports the psx_syscall() pseudo function which takes 1 to  7  arguments,
       depending  on  the needs of the caller. The macrology pads out the call to actually use psx_syscall3() or
       psx_syscall6() with zeros filling the missing arguments. While using this in source  code  will  make  it
       appear  clean,  the  actual  code  footprint  is  larger.  You  are  encouraged  to use the more explicit
       psx_syscall3() and psx_syscall6() functions as needed.

       psx_set_sensitivity() changes the  behavior  of  the  mirrored  system  calls:  PSX_IGNORE  ensures  that
       differences  are  ignored  (the default behavior); PSX_WARNING prints a stderr notification about how the
       results differ; and PSX_ERROR prints the error details and generates a SIGSYS signal.

RETURN VALUE

       The return value for system call functions is generally the value returned by the kernel, or  -1  in  the
       case  of  an  error.  In  such  cases errno(3) is set to the detailed error value. The psx_syscall3() and
       psx_syscall6() functions attempt a single threaded system call and return immediately in the case  of  an
       error. Should this call succeed, then the same system calls are executed from a signal handler on each of
       the other threads of the process.

CONFORMING TO

       The needs of libcap(3) for POSIX semantics of capability manipulation. You can read more about  why  this
       is needed here:

       https://sites.google.com/site/fullycapable/who-ordered-libpsx

REPORTING BUGS

       The  libpsx  library  is  distributed  from https://sites.google.com/site/fullycapable/ where the release
       notes may already cover recent issues.  Please report newly discovered bugs via:

       https://bugzilla.kernel.org/buglist.cgi?component=libcap&list_id=1090757

SEE ALSO

       libcap(3), pthreads(7) and nptl(7).

                                                   2021-12-12                                          LIBPSX(3)