Question

I have a kernel module that needs to have access to the kernel’s _stext and _etext symbols. For some reason, even though the kernel has definitions for these symbols (/proc/kallsyms), whenever I load my module I get

mymodule: Unknown symbol _etext (err 0)
mymodule: Unknown symbol _stext (err 0)

I’m not running a tainted kernel, and everything else in the module seems to link ok. Are there module or license restrictions on linking to _stext and _etext? Is there a substitute definition in the kernel that I could use for the beginning and end of code memory?

Update:

I’m looking at the kernel source, and in kallsyms.c, there’s this:

static int read_symbol_tr(const char *sym, unsigned long long addr)
{
size_t i;
struct text_range *tr;

for (i = 0; i < ARRAY_SIZE(text_ranges); ++i) {
    tr = &text_ranges[i];

    if (strcmp(sym, tr->stext) == 0) {
        tr->start = addr;
        return 0;
    } else if (strcmp(sym, tr->etext) == 0) {
        tr->end = addr;
        return 0;
    }
}

return 1;
}

Does this mean that the _etext and _stext entries that I’m seeing the /proc/kallsyms are fake symbols that a kernel module can’t actually link to?

Answer 1

The kernel module linker will only link symbols that have been “exported” via an additional mechanism: the symbol must be exported with EXPORT_SYMBOL() or EXPORT_SYMBOL_GPL(). (The _GPL variant indicates that the kernel developers are labeling that an internal interface and thus available only to GPL-licensed code.) I cannot find exports for either _stext or _etext on my kernel sources, so I don’t believe they are available to you to use.

What are you trying to accomplish?