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

psx_syscall3, psx_syscall6, psx_set_sensitivity - POSIX semantics for system calls

#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

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.

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.

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

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:

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

2021-12-12