ftbfs with LLVM 21:

[...]
[ 60%] Building CXX object
src/cc/CMakeFiles/bpf-static.dir/usdt/usdt_args.cc.o
cd /build/reproducible-path/bpfcc-0.35.0+ds/obj-aarch64-linux-gnu/src/cc
&& /usr/bin/c++ -DEXPORT_USDT -DHAVE_EXTERNAL_LIBBPF -DHAVE_LIBLZMA
-I/usr/lib/llvm-21/include/../tools/clang/include 
-I/build/reproducible-path/bpfcc-0.35.0+ds/src 
-I/build/reproducible-path/bpfcc-0.35.0+ds/obj-aarch64-linux-gnu/src 
-I/build/reproducible-path/bpfcc-0.35.0+ds/obj-aarch64-linux-gnu/src/cc 
-I/build/reproducible-path/bpfcc-0.35.0+ds/src/cc 
-I/build/reproducible-path/bpfcc-0.35.0+ds/obj-aarch64-linux-gnu/src/cc/frontends/b 
-I/build/reproducible-path/bpfcc-0.35.0+ds/src/cc/frontends/b 
-I/build/reproducible-path/bpfcc-0.35.0+ds/src/cc/frontends/clang 
-I/usr/lib/llvm-21/include 
-I/build/reproducible-path/bpfcc-0.35.0+ds/src/cc/compat -g -O2 
-ffile-prefix-map=/build/reproducible-path/bpfcc-0.35.0+ds=. 
-fstack-protector-strong -fstack-clash-protection -Wformat 
-Werror=format-security -mbranch-protection=standard -Wdate-time 
-D_FORTIFY_SOURCE=2 -DCUSTOM_MACRO=true -Wall  -fno-rtti -fPIC 
-DBCC_PROG_TAG_DIR='"/var/tmp/bcc"' -Wno-unused-result -std=gnu++17 
-D_GNU_SOURCE -DEXPERIMENTAL_KEY_INSTRUCTIONS -D__STDC_CONSTANT_MACROS 
-D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -MD -MT 
src/cc/CMakeFiles/bpf-static.dir/usdt/usdt_args.cc.o -MF
CMakeFiles/bpf-static.dir/usdt/usdt_args.cc.o.d -o
CMakeFiles/bpf-static.dir/usdt/usdt_args.cc.o -c
/build/reproducible-path/bpfcc-0.35.0+ds/src/cc/usdt/usdt_args.cc
/build/reproducible-path/bpfcc-0.35.0+ds/src/cc/frontends/clang/loader.cc:
In member function ‘int
ebpf::ClangLoader::do_compile(std::unique_ptr<llvm::Module>*,
ebpf::TableStorage&, bool, const std::vector<const char*,
std::allocator<const char*> >&, const std::vector<const char*,
std::allocator<const char*> >&, const std::string&, const
std::unique_ptr<llvm::MemoryBuffer>&, const std::string&,
ebpf::ProgFuncInfo&, std::string&, bool, const std::string&,
ebpf::fake_fd_map_def&, std::map<std::__cxx11::basic_string<char>,
std::vector<std::__cxx11::basic_string<char> > >&)’:
/build/reproducible-path/bpfcc-0.35.0+ds/src/cc/frontends/clang/loader.cc:400:73:
error: no matching function for call to
‘clang::TextDiagnosticPrinter::TextDiagnosticPrinter(llvm::raw_fd_ostream&,
clang::DiagnosticOptions*)’
   400 |   auto diag_client = new TextDiagnosticPrinter(llvm::errs(),
&*diag_opts);
       |
         ^
/build/reproducible-path/bpfcc-0.35.0+ds/src/cc/frontends/clang/loader.cc:400:73:
note: there is 1 candidate
In file included from
/build/reproducible-path/bpfcc-0.35.0+ds/src/cc/frontends/clang/loader.cc:47:
/usr/lib/llvm-21/include/clang/Frontend/TextDiagnosticPrinter.h:41:3:
note: candidate 1:
‘clang::TextDiagnosticPrinter::TextDiagnosticPrinter(llvm::raw_ostream&,
clang::DiagnosticOptions&, bool)’
    41 |   TextDiagnosticPrinter(raw_ostream &os, DiagnosticOptions
&DiagOpts,
       |   ^~~~~~~~~~~~~~~~~~~~~
/usr/lib/llvm-21/include/clang/Frontend/TextDiagnosticPrinter.h:41:61:
note: no known conversion for argument 2 from
‘clang::DiagnosticOptions*’ to ‘clang::DiagnosticOptions&’
    41 |   TextDiagnosticPrinter(raw_ostream &os, DiagnosticOptions
&DiagOpts,
       |
~~~~~~~~~~~~~~~~~~~^~~~~~~~
make[3]: Leaving directory
'/build/reproducible-path/bpfcc-0.35.0+ds/obj-aarch64-linux-gnu'
[ 60%] Built target api-objects
[ 61%] Building CXX object
src/cc/CMakeFiles/bpf-shared.dir/usdt/usdt_args.cc.o
cd /build/reproducible-path/bpfcc-0.35.0+ds/obj-aarch64-linux-gnu/src/cc
&& /usr/bin/c++ -DEXPORT_USDT -DHAVE_EXTERNAL_LIBBPF -DHAVE_LIBLZMA
-Dbpf_shared_EXPORTS -I/usr/lib/llvm-21/include/../tools/clang/include 
-I/build/reproducible-path/bpfcc-0.35.0+ds/src 
-I/build/reproducible-path/bpfcc-0.35.0+ds/obj-aarch64-linux-gnu/src 
-I/build/reproducible-path/bpfcc-0.35.0+ds/obj-aarch64-linux-gnu/src/cc 
-I/build/reproducible-path/bpfcc-0.35.0+ds/src/cc 
-I/build/reproducible-path/bpfcc-0.35.0+ds/obj-aarch64-linux-gnu/src/cc/frontends/b 
-I/build/reproducible-path/bpfcc-0.35.0+ds/src/cc/frontends/b 
-I/build/reproducible-path/bpfcc-0.35.0+ds/src/cc/frontends/clang 
-I/usr/lib/llvm-21/include 
-I/build/reproducible-path/bpfcc-0.35.0+ds/src/cc/compat -g -O2 
-ffile-prefix-map=/build/reproducible-path/bpfcc-0.35.0+ds=. 
-fstack-protector-strong -fstack-clash-protection -Wformat 
-Werror=format-security -mbranch-protection=standard -Wdate-time 
-D_FORTIFY_SOURCE=2 -DCUSTOM_MACRO=true -Wall  -fno-rtti -fPIC 
-DBCC_PROG_TAG_DIR='"/var/tmp/bcc"' -Wno-unused-result -std=gnu++17 
-fPIC -D_GNU_SOURCE -DEXPERIMENTAL_KEY_INSTRUCTIONS 
-D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS 
-MD -MT src/cc/CMakeFiles/bpf-shared.dir/usdt/usdt_args.cc.o -MF 
CMakeFiles/bpf-shared.dir/usdt/usdt_args.cc.o.d -o
CMakeFiles/bpf-shared.dir/usdt/usdt_args.cc.o -c
/build/reproducible-path/bpfcc-0.35.0+ds/src/cc/usdt/usdt_args.cc
In file included from /usr/lib/llvm-21/include/clang/Basic/FileManager.h:22,
                  from
/build/reproducible-path/bpfcc-0.35.0+ds/src/cc/frontends/clang/loader.cc:35:
/usr/lib/llvm-21/include/llvm/ADT/IntrusiveRefCntPtr.h: In instantiation
of ‘static void llvm::IntrusiveRefCntPtrInfo<T>::retain(T*) [with T =
clang::DiagnosticOptions]’:
/usr/lib/llvm-21/include/llvm/ADT/IntrusiveRefCntPtr.h:228:40:
required from ‘void llvm::IntrusiveRefCntPtr<T>::retain() [with T =
clang::DiagnosticOptions]’
   228 |       IntrusiveRefCntPtrInfo<T>::retain(Obj);
       |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
/usr/lib/llvm-21/include/llvm/ADT/IntrusiveRefCntPtr.h:180:43:
required from ‘llvm::IntrusiveRefCntPtr<T>::IntrusiveRefCntPtr(T*) [with
T = clang::DiagnosticOptions]’
   180 |   IntrusiveRefCntPtr(T *obj) : Obj(obj) { retain(); }
       |                                           ^~~~~~
/build/reproducible-path/bpfcc-0.35.0+ds/src/cc/frontends/clang/loader.cc:399:74:
   required from here
   399 |   IntrusiveRefCntPtr<DiagnosticOptions> diag_opts(new
DiagnosticOptions());
       |
          ^
/usr/lib/llvm-21/include/llvm/ADT/IntrusiveRefCntPtr.h:163:37: error:
‘class clang::DiagnosticOptions’ has no member named ‘Retain’
   163 |   static void retain(T *obj) { obj->Retain(); }
       |                                ~~~~~^~~~~~

Control: tags -1 patch

Looks like there's a patch for this upstream:

https://github.com/iovisor/bcc/commit/8c5c96ad3beeed2fa827017f451a952306826974

Cheers,
Emilio

I tested the upstream patch with clang 19, and it worked just fine
here.  I pushed the patch to salsa.

diff --git a/debian/patches/0002-clang-21.patch b/debian/patches/0002-clang-21.patch
new file mode 100644
index 00000000..f8e7eb04
--- /dev/null
+++ b/debian/patches/0002-clang-21.patch
@@ -0,0 +1,46 @@
+commit 8c5c96ad3beeed2fa827017f451a952306826974
+Author: yonghong-song <ys114321@gmail.com>
+Date:   Mon Jul 14 20:21:59 2025 -0700
+Origin: https://github.com/iovisor/bcc/commit/8c5c96ad3beeed2fa827017f451a952306826974
+Bug-Debian: https://bugs.debian.org/1122606
+Forwarded: not-needed
+Last-Update: 2025-12-20
+
+    Fix a build failure with clang21 (#5369)
+
+    The build error message:
+      src/cc/frontends/clang/loader.cc:400:73: error: no matching function for
+       call to ‘clang::TextDiagnosticPrinter::TextDiagnosticPrinter(
+         llvm::raw_fd_ostream&, clang::DiagnosticOptions*)’
+      400 |   auto diag_client = new TextDiagnosticPrinter(llvm::errs(), &*diag_opts);
+          |                                                                         ^
+    The llvm commit
+      https://github.com/llvm/llvm-project/pull/139584
+    caused the build failure.
+
+    Adjust the code properly and the error is fixed.
+
+diff --git a/src/cc/frontends/clang/loader.cc b/src/cc/frontends/clang/loader.cc
+index 07dc9d6a..6f8387aa 100644
+--- a/src/cc/frontends/clang/loader.cc
++++ b/src/cc/frontends/clang/loader.cc
+@@ -396,11 +396,19 @@ int ClangLoader::do_compile(
+                     flags_cstr_rem.end());
+
+   // set up the error reporting class
++#if LLVM_VERSION_MAJOR >= 21
++  DiagnosticOptions diag_opts;
++  auto diag_client = new TextDiagnosticPrinter(llvm::errs(), diag_opts);
++
++  IntrusiveRefCntPtr<DiagnosticIDs> DiagID(new DiagnosticIDs());
++  DiagnosticsEngine diags(DiagID, diag_opts, diag_client);
++#else
+   IntrusiveRefCntPtr<DiagnosticOptions> diag_opts(new DiagnosticOptions());
+   auto diag_client = new TextDiagnosticPrinter(llvm::errs(), &*diag_opts);
+
+   IntrusiveRefCntPtr<DiagnosticIDs> DiagID(new DiagnosticIDs());
+   DiagnosticsEngine diags(DiagID, &*diag_opts, diag_client);
++#endif
+
+   // set up the command line argument wrapper
+
diff --git a/debian/patches/series b/debian/patches/series
index 21b3c377..6a302de5 100644
--- a/debian/patches/series
+++ b/debian/patches/series
@@ -3,3 +3,4 @@ fix-install-path.patch
 2002_fix_netqtop.c_path.patch
 2003-libbpf-tools-debian.patch
 0001-Cleanup-existing-temporary-kernel-headers-path.patch
+0002-clang-21.patch

I am uploading the patch that Petter has pushed as NMU to DELAYED/5.
The changelog entry is also pushed to git.

We believe that the bug you reported is fixed in the latest version of
bpfcc, which is due to be installed in the Debian FTP archive.

A summary of the changes between this version and the previous one is
attached.

Thank you for reporting the bug, which will now be closed.  If you
have further comments please address them to 1122606@bugs.debian.org,
and the maintainer will reopen the bug report if appropriate.

Debian distribution maintenance software
pp.
Bastian Germann <bage@debian.org> (supplier of updated bpfcc package)

(This message was generated automatically at their request; if you
believe that there is a problem with it please contact the archive
administrators by mailing ftpmaster@ftp-master.debian.org)
Format: 1.8
Date: Mon, 29 Dec 2025 20:57:31 +0100
Source: bpfcc
Architecture: source
Version: 0.35.0+ds-1.1
Distribution: unstable
Urgency: medium
Maintainer: Ritesh Raj Sarraf <rrs@debian.org>
Changed-By: Bastian Germann <bage@debian.org>
Closes: 1122606
Changes:
 bpfcc (0.35.0+ds-1.1) unstable; urgency=medium
 .
   * Non-maintainer upload
 .
   [ Petter Reinholdtsen ]
   * [ade87c1] Fixed clang 21 build problem (Closes: #1122606)
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