Question

I’m studying the mechanism of module loading inside the linux kernel, and I’m stuck at the meaning of the field strmap inside the struct load_info.

The struct is defined in kernel/module.c and the field is filled in kernel/module.c. I know it is a bitmap, but I can’t figure out the meaning of the individual bits.

Answer 1

(I couldn’t find anything on LWN. This is a partial analysis of the code. Note that strmap existed as a separate variable before struct load_info was introduced.)

strtab contains a bunch of null-terminated strings concatenated together. E.g. it might contain this, where . stands for a null character:

foo.bar.quux.corge.
0123456789012345678

In layout_symtab, strmap is used to determine which chunks of strtab correspond to core symbols. The loop sets the jth bit of strtab whenever the jth character is part of the name of a core symbol. For example, given the table above, if bar and corge are core symbols then bits 4–7 and 13–18 are set to 1.

In add_kallsyms, the second loop computes the total length of the names of the core symbols, and the third loop copies the core symbols into a new table mod->symtab by copying only the bytes of strtab for which the corresponding bit in strmap is set. In the example above, the new table would contain bar.corge..