On 1/13/21 12:18 AM, Alexei Starovoitov wrote:
On Tue, Jan 12, 2021 at 8:30 AM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
On 1/12/21 8:55 AM, Andrii Nakryiko wrote:
Add support for directly accessing kernel module variables from BPF programs
using special ldimm64 instructions. This functionality builds upon vmlinux
ksym support, but extends ldimm64 with src_reg=BPF_PSEUDO_BTF_ID to allow
specifying kernel module BTF's FD in insn[1].imm field.
During BPF program load time, verifier will resolve FD to BTF object and will
take reference on BTF object itself and, for module BTFs, corresponding module
as well, to make sure it won't be unloaded from under running BPF program. The
mechanism used is similar to how bpf_prog keeps track of used bpf_maps.
One interesting change is also in how per-CPU variable is determined. The
logic is to find .data..percpu data section in provided BTF, but both vmlinux
and module each have their own .data..percpu entries in BTF. So for module's
case, the search for DATASEC record needs to look at only module's added BTF
types. This is implemented with custom search function.
Acked-by: Yonghong Song <yhs@xxxxxx>
Acked-by: Hao Luo <haoluo@xxxxxxxxxx>
Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
[...]
+
+struct module *btf_try_get_module(const struct btf *btf)
+{
+ struct module *res = NULL;
+#ifdef CONFIG_DEBUG_INFO_BTF_MODULES
+ struct btf_module *btf_mod, *tmp;
+
+ mutex_lock(&btf_module_mutex);
+ list_for_each_entry_safe(btf_mod, tmp, &btf_modules, list) {
+ if (btf_mod->btf != btf)
+ continue;
+
+ if (try_module_get(btf_mod->module))
+ res = btf_mod->module;
One more thought (follow-up would be okay I'd think) ... when a module references
a symbol from another module, it similarly needs to bump the refcount of the module
that is owning it and thus disallowing to unload for that other module's lifetime.
That usage dependency is visible via /proc/modules however, so if unload doesn't work
then lsmod allows a way to introspect that to the user. This seems to be achieved via
resolve_symbol() where it records its dependency/usage. Would be great if we could at
some point also include the BPF prog name into that list so that this is more obvious.
Wdyt?
I thought about it as well, but plenty of kernel things just grab the ref of ko
and don't add any way to introspect what piece of kernel is holding ko.
So this case won't be the first.
Also if we add it for bpf progs it could be confusing in lsmod.
Since it currently only shows other ko-s in there.
Long ago I had an awk script to parse that output to rmmod dependent modules
before rmmoding the main one. If somebody doing something like this
bpf prog names in the same place may break things.
So I think there are more cons than pros.
Hm, true that scripting could break in this case if we were to add bpf prog names in
there. :/ I don't have a better suggestion atm.. we could potentially add something
for the bpf prog info dump via bpftool, but it's a non-obvious location to people who
are used to check deps via lsmod. Also true that we bump ref from plenty of other
locations where it's not directly shown either apart from just the refcnt (e.g. socket
using tcp congctl module etc).
