From 4f7aa145b796c044526c93f390e68f3b56a1b30a Mon Sep 17 00:00:00 2001 From: David Faust Date: Wed, 7 Dec 2022 11:47:26 -0800 Subject: [PATCH] btf: fix 'extern const void' variables [PR106773] The eBPF loader expects to find BTF_KIND_VAR records for references to extern const void symbols. We were mistakenly identifing these as unsupported types, and as a result skipping emitting VAR records for them. In addition, the internal DWARF representation from which BTF is produced does not generate 'const' modifier DIEs for the void type, which meant in BTF the 'const' qualifier was dropped for 'extern const void' variables. This patch also adds support for generating a const void type in BTF to correct emission for these variables. PR target/106773 gcc/ * btfout.cc (btf_collect_datasec): Correct size of void entries. (btf_dvd_emit_preprocess_cb): Do not skip emitting variables which refer to void types. (btf_init_postprocess): Create 'const void' type record if needed and adjust variables to refer to it as appropriate. gcc/testsuite/ * gcc.dg/debug/btf/btf-pr106773.c: New test. --- gcc/btfout.cc | 44 ++++++++++++++++++++++++--- gcc/testsuite/gcc.dg/debug/btf/btf-pr106773.c | 25 +++++++++++++++ 2 files changed, 65 insertions(+), 4 deletions(-) create mode 100644 gcc/testsuite/gcc.dg/debug/btf/btf-pr106773.c diff --git a/gcc/btfout.cc b/gcc/btfout.cc index 677e832..204b11d 100644 --- a/gcc/btfout.cc +++ b/gcc/btfout.cc @@ -350,6 +350,8 @@ btf_collect_datasec (ctf_container_ref ctfc) tree size = DECL_SIZE_UNIT (node->decl); if (tree_fits_uhwi_p (size)) info.size = tree_to_uhwi (size); + else if (VOID_TYPE_P (TREE_TYPE (node->decl))) + info.size = 1; /* Offset is left as 0 at compile time, to be filled in by loaders such as libbpf. */ @@ -441,7 +443,7 @@ btf_dvd_emit_preprocess_cb (ctf_dvdef_ref *slot, ctf_container_ref arg_ctfc) return 1; /* Do not add variables which refer to unsupported types. */ - if (btf_removed_type_p (var->dvd_type)) + if (!voids.contains (var->dvd_type) && btf_removed_type_p (var->dvd_type)) return 1; arg_ctfc->ctfc_vars_list[num_vars_added] = var; @@ -1075,15 +1077,49 @@ btf_init_postprocess (void) { ctf_container_ref tu_ctfc = ctf_get_tu_ctfc (); - size_t i; - size_t num_ctf_types = tu_ctfc->ctfc_types->elements (); - holes.create (0); voids.create (0); num_types_added = 0; num_types_created = 0; + /* Workaround for 'const void' variables. These variables are sometimes used + in eBPF programs to address kernel symbols. DWARF does not generate const + qualifier on void type, so we would incorrectly emit these variables + without the const qualifier. + Unfortunately we need the TREE node to know it was const, and we need + to create the const modifier type (if needed) now, before making the types + list. So we can't avoid iterating with FOR_EACH_VARIABLE here, and then + again when creating the DATASEC entries. */ + ctf_id_t constvoid_id = CTF_NULL_TYPEID; + varpool_node *var; + FOR_EACH_VARIABLE (var) + { + if (!var->decl) + continue; + + tree type = TREE_TYPE (var->decl); + if (type && VOID_TYPE_P (type) && TYPE_READONLY (type)) + { + dw_die_ref die = lookup_decl_die (var->decl); + if (die == NULL) + continue; + + ctf_dvdef_ref dvd = ctf_dvd_lookup (tu_ctfc, die); + if (dvd == NULL) + continue; + + /* Create the 'const' modifier type for void. */ + if (constvoid_id == CTF_NULL_TYPEID) + constvoid_id = ctf_add_reftype (tu_ctfc, CTF_ADD_ROOT, + dvd->dvd_type, CTF_K_CONST, NULL); + dvd->dvd_type = constvoid_id; + } + } + + size_t i; + size_t num_ctf_types = tu_ctfc->ctfc_types->elements (); + if (num_ctf_types) { init_btf_id_map (num_ctf_types + 1); diff --git a/gcc/testsuite/gcc.dg/debug/btf/btf-pr106773.c b/gcc/testsuite/gcc.dg/debug/btf/btf-pr106773.c new file mode 100644 index 0000000..f90fa77 --- /dev/null +++ b/gcc/testsuite/gcc.dg/debug/btf/btf-pr106773.c @@ -0,0 +1,25 @@ +/* Test BTF generation for extern const void symbols. + BTF_KIND_VAR records should be emitted for such symbols if they are used, + as well as a corresponding entry in the appropriate DATASEC record. */ + +/* { dg-do compile } */ +/* { dg-options "-O0 -gbtf -dA" } */ + +/* Expect 1 variable record only for foo, with 'extern' (2) linkage. */ +/* { dg-final { scan-assembler-times "\[\t \]0xe000000\[\t \]+\[^\n\]*btv_info" 1 } } */ +/* { dg-final { scan-assembler-times "\[\t \]0x2\[\t \]+\[^\n\]*btv_linkage" 1 } } */ + +/* { dg-final { scan-assembler-times "ascii \"foo.0\"\[\t \]+\[^\n\]*btf_string" 1 } } */ + +/* { dg-final { scan-assembler-times "0\[\t \]+\[^\n\]*bts_offset" 1 } } */ +/* { dg-final { scan-assembler-times "1\[\t \]+\[^\n\]*bts_size" 1 } } */ + +extern const void foo __attribute__((weak)) __attribute__((section (".ksyms"))); +extern const void bar __attribute__((weak)) __attribute__((section (".ksyms"))); + +unsigned long func () { + unsigned long x = (unsigned long) &foo; + + return x; +} + -- 2.7.4