On 5/12/23 7:59 PM, Yonghong Song wrote:
On 5/12/23 2:54 PM, Jiri Olsa wrote:
On Fri, May 12, 2023 at 11:59:34AM -0700, Alexei Starovoitov wrote:
On Fri, May 12, 2023 at 9:04 AM Alan Maguire
<alan.maguire@xxxxxxxxxx> wrote:
On 12/05/2023 03:51, Yafang Shao wrote:
On Wed, May 10, 2023 at 9:03 PM Alan Maguire
<alan.maguire@xxxxxxxxxx> wrote:
v1.25 of pahole supports filtering out functions with multiple
inconsistent
function prototypes or optimized-out parameters from the BTF
representation.
These present problems because there is no additional info in BTF
saying which
inconsistent prototype matches which function instance to help
guide attachment,
and functions with optimized-out parameters can lead to incorrect
assumptions
about register contents.
So for now, filter out such functions while adding BTF
representations for
functions that have "."-suffixes (foo.isra.0) but not
optimized-out parameters.
This patch assumes that below linked changes land in pahole for
v1.25.
Issues with pahole filtering being too aggressive in removing
functions
appear to be resolved now, but CI and further testing will confirm.
Signed-off-by: Alan Maguire <alan.maguire@xxxxxxxxxx>
---
scripts/pahole-flags.sh | 3 +++
1 file changed, 3 insertions(+)
diff --git a/scripts/pahole-flags.sh b/scripts/pahole-flags.sh
index 1f1f1d397c39..728d55190d97 100755
--- a/scripts/pahole-flags.sh
+++ b/scripts/pahole-flags.sh
@@ -23,5 +23,8 @@ if [ "${pahole_ver}" -ge "124" ]; then
# see PAHOLE_HAS_LANG_EXCLUDE
extra_paholeopt="${extra_paholeopt} --lang_exclude=rust"
fi
+if [ "${pahole_ver}" -ge "125" ]; then
+ extra_paholeopt="${extra_paholeopt}
--skip_encoding_btf_inconsistent_proto --btf_gen_optimized"
+fi
echo ${extra_paholeopt}
--
2.31.1
That change looks like a workaround to me.
There may be multiple functions that have the same proto, e.g.:
$ grep -r "bpf_iter_detach_map(struct bpf_iter_aux_info \*aux)"
kernel/bpf/ net/core/
kernel/bpf/map_iter.c:static void bpf_iter_detach_map(struct
bpf_iter_aux_info *aux)
net/core/bpf_sk_storage.c:static void bpf_iter_detach_map(struct
bpf_iter_aux_info *aux)
$ bpftool btf dump file /sys/kernel/btf/vmlinux | grep -B 2
bpf_iter_detach_map
[34691] FUNC_PROTO '(anon)' ret_type_id=0 vlen=1
'aux' type_id=2638
[34692] FUNC 'bpf_iter_detach_map' type_id=34691 linkage=static
We don't know which one it is in the BTF.
However, I'm not against this change, as it can avoid some issues.
In the above case, the BTF representation is consistent though.
That is, if I attach fentry progs to either of these functions
based on that BTF representation, nothing will crash.
That's ultimately what those changes are about; ensuring
consistency in BTF representation, so when a function is in
BTF we can know the signature of the function can be safely
used by fentry for example.
The question of being able to identify functions (as opposed
to having a consistent representation) is the next step.
Finding a way to link between kallsyms and BTF would allow us to
have multiple inconsistent functions in BTF, since we could map
from BTF -> kallsyms safely. So two functions called "foo"
with different function signatures would be okay, because
we'd know which was which in kallsyms and could attach
safely. Something like a BTF tag for the function that could
clarify that mapping - but just for cases where it would
otherwise be ambiguous - is probably the way forward
longer term.
Jiri's talking about this topic at LSF/MM/BPF this week I believe.
Jiri presented a few ideas during LSFMMBPF.
I feel the best approach is to add a set of addr-s to BTF
via a special decl_tag.
We can also consider extending KIND_FUNC.
The advantage that every BTF func will have one or more addrs
associated with it and bpf prog loading logic wouldn't need to do
fragile name comparison between btf and kallsyms.
pahole can take addrs from dwarf and optionally double check with
kallsyms.
Yonghong summed it up in another email discussion, pasting it in here:
So overall we have three options as kallsyms representation now:
(a) "addr module:foo:dir_a/dir_b/core.c"
(b) "addr module:foo"
(c) "addr module:foo:btf_id"
option (a):
'dir_a/dir_b/core.c' needs to be encoded in BTF.
user space either check file path or func signature
to find attach_btf_id and pass to the kernel.
kernel can find file path in BTF and then lookup
kallsyms to find addr.
option (b):
"addr" needs to be encoded in BTF.
user space checks func signature to find
attach_btf_id and pass to the kernel.
kernel can find addr in BTF and use it.
option (c):
if user can decide which function to attach, e.g.,
through func signature, then no BTF encoding
is necessary. attach_btf_id is passed to the
kernel and search kallsyms to find the matching
btf_id and 'addr' will be available then.
For option (b) and (c), user space needs to check
func signature to find which btf_id to use. If
same-name static functions having the identical
signatures, then user space would have a hard time
to differentiate. I think it should be very
rare same-name static functions in the kernel will have
identical signatures. But if we want 100% correctness,
we may need file path in which case option (a)
is preferable.
As Alexei mentioned in previous email, for such a extreme case,
if user is willing to go through extra step to check dwarf
to find and match file path, then (b) and (c) should work
perfectly as well.
Okay, it looks like this is more complex if the function signature is
the same. In such cases, current BTF dedup will merge these
functions as a single BTF func. In such cases, we could have:
decl_tag_1 ----> dedup'ed static_func
^
|
decl_tag_2 ---------
For such cases, just passing btf_id of static func to kernel
won't work since the kernel won't be able to know which
decl_tag to be associated with.
(I did a simple test with vmlinux, it looks we have
issues with decl_tag_1/decl_tag_2 -> dedup'ed static_func
as well since only one of decl_tag survives.
But this is a different issue.
)
So if we intend to add decl tag (addr or file_path), we
should not dedup static functions or generally any functions.
Current option (a) kallsyms format is under review.
option (c) also needs kallsyms change...
my thoughts so far is that I like the idea of storing functions address
in BTF (option b), because it's the easiest way
on the other hand, user would need debug info to find address for the
function
to trace.. but still just for small subset of functions that share
same name
also I like Lorenz's idea of storing BTF ID in kalsyms and verifier being
able to lookup address based on BTF ID.. seems like easier kallsyms
change,
but it would still require storing objects paths in BTF to pick up the
correct one
cc-ing other folks
jirka
