Date: Wed, 26 Apr 2023 14:12:40 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Hans Petter Selasky <hselasky@freebsd.org> Cc: Zhenlei Huang <zlei@freebsd.org>, FreeBSD CURRENT <freebsd-current@freebsd.org>, Gleb Smirnoff <glebius@freebsd.org> Subject: Re: Link modules to DYN type Message-ID: <ZEkHKJ_BRhV22gf_@kib.kiev.ua> In-Reply-To: <2bb66cac-c7f1-e45b-693a-8afbda05cfa6@freebsd.org> References: <97390FE1-1DF5-43A1-A3F4-2B945D681437@FreeBSD.org> <2bb66cac-c7f1-e45b-693a-8afbda05cfa6@freebsd.org>
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On Wed, Apr 26, 2023 at 12:55:02PM +0200, Hans Petter Selasky wrote: > On 4/26/23 12:36, Zhenlei Huang wrote: > > Hi, > > > > I'm recently working on https://reviews.freebsd.org/D39638 (sysctl(9): Enable vnet sysctl variables be loader tunable), > > the changes to `sys/kern/link_elf_obj.c` are runtime tested, but not those to `sys/kern/link_elf.c` . > > > > After some hacking I realized that `link_elf.c` is for EXEC (Executable file) or DYN (Shared object file), and `link_elf_obj.c` is > > for REL (Relocatable file). > > > > ``` > > /* link_elf.c */ > > static int > > link_elf_load_file(linker_class_t cls, const char* filename, > > linker_file_t* result) > > { > > ... > > if (hdr->e_type != ET_EXEC && hdr->e_type != ET_DYN) { > > error = ENOSYS; > > goto out; > > } > > ... > > } > > > > > > /* link_elf_obj.c */ > > static int > > link_elf_load_file(linker_class_t cls, const char *filename, > > linker_file_t *result) > > { > > ... > > if (hdr->e_type != ET_REL) { > > error = ENOSYS; > > goto out; > > } > > ... > > } > > ``` > > > > Run the following snip: > > ``` > > # find /boot/kernel -type f -name "*.ko" -exec readelf -h {} \; | grep Type > > ``` > > shows that all the kernel modules' types are `REL (Relocatable file)`. > > > > I guess if some module such as if_bridge is linked to DYN type, then I can do runtime for the changes to `sys/kern/link_elf.c`. > > > > I'm not familiar with elf and linkers, is that ( compile module and link it to DYN type ) possible ? Module file type (shared object vs. object file) depends on architecture. For amd64 modules are objects, while kernel is shared library. For arm64 (and all other arches, I believe) modules and kernels are shared libraries. I think you can link amd64 module as shared object, but this require enough hacking of the build infrastructure. At least I am not aware of a simple knob to switch the produced type. > > > > Hi, > > I don't have an answer for you either, but I have seen in the past, loading > kernel modules behaves a bit like libraries, in the following regard: > > If two kernel modules define the same global symbol, then no warning is > given and the first loaded symbol definition (I think) is used to resolve > that symbol for all kernel modules, regardless of the prototype. Probably we > should not allow this. That's why building LINT is a good thing, to avoid > this issue. No, in-kernel linker does not behave this way. Modules need to contain explicit reference to all modules they depend upon, using the MODULE_DEPEND() macro. Only symbols from the dependencies are resolved. All modules get an implicit reference to kernel. > > Even if we don't have C++ support in the FreeBSD kernel, defining symbol > names the way C++ does for C could be nice for the kernel too, also with > regards to debugging systems. > > Many times when I don't know what is going on, I do like this: > > #include <sys/kdb.h> > > .... > > if (not too fast or my sysctl debug) { > printf("My tracer\n"); > kdb_backtrace(); > } > > Dtrace can also do this, but not during boot. Just track who is calling > those functions, and you'll probably find the answer to your question! > > --HPS > > > > > Best regards, > > Zhenlei > > >
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