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Adding SafePOSIX Implementation of Syscalls
Two main paths can be taken in implementing a new system call, defined in the Native Client section. In both these paths, a system call is defined in Lind_GlibC.
We customize the Standard C library used by the NaCl C compiler in GlibC to identify the system calls we implement in Lind and link any further references to our implementations.
First, a file in Lind_GlibC/sysdeps/nacl/ named X.c should be created for a system call X. In this file, a function to implement the system call should be defined with the name __X. This definition should be followed by weak_alias (__X, X) to denote the reference. libc_hidden_def(__X) should also be used here if the header file for the function has libc_hidden_proto(__X).
One can also choose to add both libc_hidden_proto and libc_hidden_def to the relevant files if libc_hidden_proto is not present in the header. For the functionality of these functions, one can look at the libc-symbols.h file of the original Glibc source code.
This file's full path (sysdeps/nacl/X.c) should then be added to the override_list in make_sysd_rules.py to make sure that this function is used.
This __X function should reference a function that can call the NACL_SYSCALL function. Such functions can be implemented in either sysdeps/nacl/lind_syscalls.c or sysdeps/nacl/irt_syscalls.c (after being defined in the respective header files).
In irt_syscalls.c, the convention is to implement this function as nacl_irt_X after defining the function as *__nacl_irt_X in both the header and the .c file. This is followed by an assignment: __nacl_irt_X = nacl_irt_X at the bottom of irt_syscalls.c. These function are normally used to call NACL_SYSCALL with a custom system call name.
The function should be defined nacl_syscalls.h after the proper assignment of the syscall number.
The functions defined in irt_syscalls.c (__nacl_irt_X) should then be added to the GLIBC_PRIVATE part in Lind_GlibC/elf/Versions alongside other system calls to ensure their interception.
We make changes in native_client to define the behavior of the system calls in Lind. The general principle is that we make the security checks related to the information regarding the state of the NaCl session before initiating system calls here. We then define the procedures for the calling of Posix implementations and thereby link our Restricted Python syscall implementations to Native Client.
The changes needed to be done in the Native Client is completely dependent on the preferred development path:
In this path, the NACL_SYSCALL(lind_api) reaches the NaClSysLindSyscall function in native_client/src/trusted/service_runtime/lind_syscalls.c file directly. In this function; the parameters are parsed and are combined with the cage_id. Then, the CallPythonFunc function in native_client/src/shared/platform/lind_platform.c is called with the proper parameters. This function carries out the RPC, directly relaying the system call number coming from GlibC to the RePy alongside the proper arguments format.
No changes are necessary for any of the files in Native Client for this path.
First, the name of the implemented syscall in GlibC should be defined as the number assigned to the syscall in GlibC in the file native_client/src/trusted/service_runtime/include/bits/nacl_syscalls.h to allow for the use of the same name macro here.
Then, the NACL_SYSCALL being implemented should be added to the SYSCALL_LIST in native_client/src/trusted/service_runtime/nacl_syscall_handlers_gen.py, denoting the inputs used and linking the name on the GlibC side to the syscall name in NaCl.
Then, the referred NaCl function should be defined in native_client/src/trusted/service_runtime/nacl_syscall_common.h and implemented in native_client/src/trusted/service_runtime/nacl_syscall_common.c. In the implementation here, the necessary security check should be made, and parsing of information that must be one in C must be carried out. Then, a call must be made to one of the functions pointed by the NaClDescVtbl (more information available at native_client/src/trusted/desc/nacl_desc_base.h ) if the syscall is related to one of the functions pointed there. Such a call goes through many loops but finally lands on the functions calling the Posix implementations. Then, the result should be parsed and returned in the proper format. Otherwise, a direct calling of "lind_" functions lind_platform.h/lind_platform.c should be carried out.
Lastly, the implementation of functions linking the NaCl to Posix should be made. For this, functions with the name lind_X (for a syscall named X) should be defined in native_client/src/shared/platform/lind_platform.h that include their standard parameters and the cage id. Also, macros for the name of the syscalls (LIND_safe_fs_X / LIND_safe_net_X) should be defined here to match the numbers assigned to the GlibC/NaCl macros.
We implement syscalls here. For more details about SafePOSIX-Rust interface, please check the SafePOSIX wiki.
This file is where RPC is accepted and syscalls are delivered to the dispatcher. Have the new syscall included here:
safeposix-rust/src/safeposix/dispatcher.rs
The number assigned to the system call here must match the one assigned in GlibC. Add the syscall's Lind implementation here:
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safeposix-rust/src/safeposix/syscalls/fs_calls.rsOR -
safeposix-rust/src/safeposix/syscalls/net_calls.rsOR safeposix-rust/src/safeposix/syscalls/sys_calls.rs
Lind-GlibC/sysdeps/nacl/X.cLind-GlibC/make_sysd_rules.pyLind-GlibC/elf/VersionsLind-GlibC/sysdeps/nacl/irt_syscalls.hLind-GlibC/sysdeps/nacl/irt_syscalls.cLind-GlibC/sysdeps/nacl/nacl_syscalls.h
native_client/src/trusted/service_runtime/include/bits/nacl_syscalls.hnative_client/src/trusted/service_runtime/nacl_syscall_handlers_gen.pynative_client/src/trusted/service_runtime/nacl_syscall_common.cnative_client/src/trusted/service_runtime/nacl_syscall_common.hnative_client/src/shared/platform/lind_platform.cnative_client/src/shared/platform/lind_platform.h
For reference:
native_client/src/trusted/desc/nacl_desc_base.h
safeposix-rust/src/safeposix/dispatcher.rssafeposix-rust/src/safeposix/syscalls/fs_calls.rssafeposix-rust/src/safeposix/syscalls/net_calls.rs