libpsx(3) — Linux manual page

LIBPSX(3)               Linux Programmer's Manual              LIBPSX(3)

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);
       void psx_load_syscalls(long int (**syscall_fn)(long int,
                                           long int, long int, long int),
                              long int (**syscall6_fn)(long int,
                                           long int, long int, long int,
                                           long int, long int, long int));

       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 (which ultimately invokes
       __psx_syscall()) 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.

       psx_load_syscalls() can be used to set caller defined function
       pointers for invoking 3 and 6 argument syscalls. This function
       can be used to configure a library, or program to change behavior
       when linked against libpsx.  Indeed, libcap uses this function
       from libpsx to override its thread scoped default system call
       based API. When linked with libpsx, libcap can operate on all the
       threads of a multithreaded program to operate with POSIX
       semantics.

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).

COLOPHON

       This page is part of the libcap (capabilities commands and
       library) project.  Information about the project can be found at
       ⟨https://git.kernel.org/pub/scm/libs/libcap/libcap.git/⟩.  If you
       have a bug report for this manual page, send it to
       morgan@kernel.org (please put "libcap" in the Subject line).
       This page was obtained from the project's upstream Git repository
       ⟨https://git.kernel.org/pub/scm/libs/libcap/libcap.git/⟩ on
       2024-06-14.  (At that time, the date of the most recent commit
       that was found in the repository was 2024-05-18.)  If you
       discover any rendering problems in this HTML version of the page,
       or you believe there is a better or more up-to-date source for
       the page, or you have corrections or improvements to the
       information in this COLOPHON (which is not part of the original
       manual page), send a mail to man-pages@man7.org

                               2021-12-12                      LIBPSX(3)

Pages that refer to this page: cap_get_proc(3), cap_launch(3), libcap(3)

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