For vmlinux built with clang thin-lto or lto for latest bpf-next,
there exist cross cu debuginfo type references. For example,
compile unit 1:
tag 10: type A
compile unit 2:
...
refer to type A (tag 10 in compile unit 1)
I only checked a few but have seen type A may be a simple type
like "unsigned char" or a complex type like an array of base types.
I am using latest llvm trunk and bpf-next. I suspect llvm12 or
linus tree >= 5.12 rc2 should be able to exhibit the issue as well.
Both thin-lto and lto have the same issues.
Current pahole cannot handle this. It will report types cannot
be found error. Bill Wendling has attempted to fix the issue
with [1] by permitting all tags/types are hashed to the same
hash table and then process cu's one by one. This does not
really work. The reason is that each cu resolves types locally
so for the above example we may have
compile unit 1:
type A : type_id = 10
compile unit 2:
refer to type A : type A will be resolved as type id = 10
But id 10 refers to compile unit 1, we will get either out
of bound type id or incorrect one.
This patch set is a continuation of Bill's work. We still
increase the hashtable size and traverse all cu's before
recoding and finalization. But instead of creating one-to-one
mapping between debuginfo cu and pahole cu, we just create
one pahole cu, which should solve the above incorrect type
id issue.
This patch set depends on kernel patch [2]
to emit compilation flags for clang lto build so pahole
can properly discover whether to merge cu's or not.
Patch #1 and #2 are refactoring the existing code and
Patch #3 added logic to premit merging all debuginfo cu's
into one pahole cu. The detection for whether merging is
desirable is done by checking the existence of
"clang" compiler and its "lto" option in dwarf producer tag.
[1] https://lore.kernel.org/bpf/20210212211607.2890660-1-morbo@xxxxxxxxxx/
[2] https://lore.kernel.org/bpf/20210328064121.2062927-1-yhs@xxxxxx/
Changelogs:
v2 -> v3:
. change "return 1" to "return DWARF_CB_ABORT" in
cus__merge_and_process_cu().
. add kbuild/bpf link (above [2]) for kernel patch reference.
v1 -> v2:
. removed "--merge_cus" option, relied on detections on
clang compiler and its lto flags.
Yonghong Song (3):
dwarf_loader: permits flexible HASHTAGS__BITS
dwarf_loader: factor out common code to initialize a cu
dwarf_loader: permit merging all dwarf cu's for clang lto built binary
dwarf_loader.c | 209 +++++++++++++++++++++++++++++++++++++++++--------
1 file changed, 175 insertions(+), 34 deletions(-)
--
2.30.2
