#board-linux

This directory contains the necessary files for the Linux simulator filesystem. To have the same filesystem interface for the Linux simulator and RT1051 we have to translate paths. It is done by wrappers which translate path and then invoke real syscall from glibc. The advantage of this solution is that we don't need to translate the path in the application layer, everything is done by low-level wrappers. So we don't need to modify third-party libraries to translate path. However, we need to maintain glibc's versions if we change the compiler or libraries.

#How does it work

If opendir function is invoked

DIR *dir;
if ((dir = opendir (".")) == NULL) {
    perror ("Cannot open .");
    exit (1);
}
//...

it is linked to our wrapper as _iosys_opendir

DIR *_iosys_opendir(const char *dirname)
{
    __dirstream *ret{};
    if (!dirname) {
        //...
        errno = EINVAL;
        return ret;
    }
    if (vfs::redirect_to_image(dirname)) {
        //...
    }
    else {
        char tmp[PATH_MAX];
        // Path translation
        const auto newpath = vfs::npath_translate(dirname, tmp);
        // Invoke opendir from glibc
        ret = real::opendir(newpath);
    }
    return ret;
}
__asm__(".symver _iosys_opendir,opendir@GLIBC_2.2.5");

As you see it is done by .symver _iosys_opendir,opendir@GLIBC_2.2.5 which links it with a proper version of glibc function.

        +--------------+                      +--------------------------+                  +-----------------+
        |              |    invoke wrapper    | _iosys_opendir(...) {    |   invoke glibc   |                 |
        | opendir(...) |--------------------->| opendir(translated_path) |----------------->| real::opendir() |
        |              |                      | }                        |                  |                 |
        +--------------+                      +--------------------------+                  +-----------------+

#How to find correct version

Our library doesn't handle all syscalls because we don't know the exact syscall version. While changing compiler or some library it will be necessary to update the version or add a new wrapper.

Example: Let's say we have a new compiler and there is stdc++ implementation that uses a different stat syscall or different version (see Related links). We can easily find the correct version by checking symbols.

$ readelf -Ws /usr/lib/x86_64-linux-gnu/libstdc++.so.6 | grep -w stat
   176: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND stat@GLIBC_2.33 (66)

#How to update syscalls wrappers

#Update version.txt

This file is used by linker to tell which internal names to replace. Before editing check glibc version and do not duplicate symbols.

GLIBC_2.33 {
    global:
        stat;   # updated stat version inserting to GLIBC_2.33
        lstat;
        fstat;
        stat64;
        lstat64;
        fstat64;
};

#Add or update wrapper

Updating the wrapper version is easy. You need to modify only @GLIBC string according to symbols found previously.

__asm__(".symver _iosys_stat,stat@GLIBC_<version>");

However if you need to add a new wrapper the best way is just to check other implementations. But finally it should look like:

_iosys_<function_name>(...)
{
    // VFS implementation if needed
    if (...) {
        //...
    }
    else {
        // Path translation if needed
        // Invoke function_name from glibc id needed
    }
}
__asm__(".symver _iosys_<function_name>,function_name@GLIBC_<version>");

#Add declarations (if needed)

If you added a wrapper then you must add helpers for intercepting library calls in real namespace. These functions take a handle of a dynamic loaded shared object and return the address where that symbol is loaded into memory.

• __REAL_DECL(function_name) - just decltype
• __REAL_DLSYM(function_name) obtain address of a symbol in a shared object or executable

#Related

GNU C library backward compatibility

GNU C library syscalls versions

DLSYM Manual