On Tue, 21 Jul 2026, Jose E. Marchesi wrote:
>
> This BPF related patch involved dwarf2out.
> Is it OK for GCC 16?
OK.
> ---
>
> typedef chain traversal could sometimes skip an intermediate type.
> This could happen when two DIEs have a common underlying type but diverge
> due to presence of additional attribute in one case and a qualifier in
> other.
> In the example from testcase, variable "perm" typechain misses interim u16.
>
> typedef unsigned short __u16;
> typedef __u16 u16;
> __attribute__((btf_type_tag(""))) u16 a;
> const u16 perm;
>
> generates
>
> .uleb128 0x1 # (DIE (0x66) DW_TAG_variable)
> .long .LASF3 # DW_AT_name: "perm"
> # DW_AT_decl_file (1, pr125421.c)
> .byte 0xa # DW_AT_decl_line
> .byte 0xb # DW_AT_decl_column
> .long 0x2f # DW_AT_type
>
> .uleb128 0x4 # (DIE (0x2f) DW_TAG_const_type)
> .long 0x23 # DW_AT_type <-- BUG
>
> .uleb128 0x3 # (DIE (0x23) DW_TAG_typedef)
> .long .LASF5 # DW_AT_name: "__u16"
> .byte 0x1 # DW_AT_decl_file (pr125421.c)
> .byte 0x7 # DW_AT_decl_line
> .byte 0xf # DW_AT_decl_column
> .long 0x34 # DW_AT_type
>
> .uleb128 0x5 # (DIE (0x34) DW_TAG_base_type)
> .byte 0x2 # DW_AT_byte_size
> .byte 0x5 # DW_AT_encoding
> .long .LASF6 # DW_AT_name: "short int"
>
> What makes this issue worse is depending on the order in which the types
> are specified in source, and processed, the problem may or maynot show up.
> So in the test code above if lines 3 and 4 are swppaed, "perm" gets the
> missing type u16 as expected.
>
> The issue is in modified_type_die (), get_qualified_type (type) returned
> pointer may not be identical to dtype: TREE_TYPE (TYPE_NAME (qualified_type))
> while having the same underlying base type. And due to the failed
> pointer identity test, subsequent usage of DECL_ORIGINAL_TYPE () for recursive
> chain processing can peel away the needed type.
>
> The implications are for a multi CU build, structurally similar but
> non identical variants of types (due to an embedded member getting a
> different chained typdef) can be generated and accumulate at link time
> in the final .debug_info.
>
> This would be OK / unnoticed for usual dwarf debugging purposes.
> However in BFP workflow: kernel binary linking invokes pahole to process
> the dwarf and dedup it for btf generation (gcc can emit btf directly but
> thats not been done currently for other reasons). The slightly different
> variations of same structure due to thi issue cause combinatiorial explosion
> in pahole dedup processing trying to match and failing repeatedly in what
> seems like infinite recursion. The problem showed up and was excerbated
> when trying to enable attr btf_type_tag in kernel with gcc for the first
> time as that is the key ingredient to trigger the latent issue.
>
> Fix is to handle qualified_type != dtype as if they were identical when
> they are different variant nodes of the same underlying typedef.
>
> Bootstrapped and regtested on x86 and aarch64.
>
> PR debug/125421
>
> gcc/ChangeLog:
>
> * dwarf2out.cc (modified_type_die): Handle qualified_type being
> a variant of dtype as if they were identical to avoid peeling a
> typdef.
> For named types, use dentry unconditionally as that is valid for
> both cases of qualified_type == and != dentry.
>
> gcc/testsuite/ChangeLog:
>
> * gcc.dg/debug/dwarf2/pr125421.c: New Test.
>
> Signed-off-by: Vineet Gupta <[email protected]>
> (cherry picked from commit 3e2885588fe0e5273a50c471d41c61ee9aefc269)
> ---
> gcc/dwarf2out.cc | 15 ++++++++++++---
> gcc/testsuite/gcc.dg/debug/dwarf2/pr125421.c | 18 ++++++++++++++++++
> 2 files changed, 30 insertions(+), 3 deletions(-)
> create mode 100644 gcc/testsuite/gcc.dg/debug/dwarf2/pr125421.c
>
> diff --git a/gcc/dwarf2out.cc b/gcc/dwarf2out.cc
> index bf342e468bf..9d6ca06067f 100644
> --- a/gcc/dwarf2out.cc
> +++ b/gcc/dwarf2out.cc
> @@ -13997,7 +13997,16 @@ modified_type_die (tree type, int cv_quals, tree
> type_attrs, bool reverse,
> tree dtype = TREE_TYPE (name);
>
> /* Skip the typedef for base types with DW_AT_endianity, no big deal.
> */
> - if (qualified_type == dtype && !reverse_type)
> + if (!reverse_type
> + && (qualified_type == dtype
> + /* Pointer identity check above might fail when qualified_type
> + is a different variant node of the same typedef yet requires
> + the same handling as if they matched (see PR/125421).
> + Skip this when btf_type_tag attributes are present, as those
> + need to be handled in the else branch below. */
> + || (TYPE_NAME (qualified_type) == name
> + && TYPE_QUALS (qualified_type) == TYPE_QUALS (dtype)
> + && !lookup_attribute ("btf_type_tag", type_attrs))))
> {
> tree origin = decl_ultimate_origin (name);
>
> @@ -14009,8 +14018,8 @@ modified_type_die (tree type, int cv_quals, tree
> type_attrs, bool reverse,
> reverse, context_die);
>
> /* For a named type, use the typedef. */
> - gen_type_die (qualified_type, context_die);
> - return lookup_type_die (qualified_type);
> + gen_type_die (dtype, context_die);
> + return lookup_type_die (dtype);
> }
> else
> {
> diff --git a/gcc/testsuite/gcc.dg/debug/dwarf2/pr125421.c
> b/gcc/testsuite/gcc.dg/debug/dwarf2/pr125421.c
> new file mode 100644
> index 00000000000..810afeae739
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/debug/dwarf2/pr125421.c
> @@ -0,0 +1,18 @@
> +/* Verify the typedef chains are preserved.
> + DW_TAG_const_type for "const u16" chains to u16 typedef and doesnot
> + skip to underlying __u16 typedef */
> +
> +/* { dg-do compile } */
> +/* { dg-options "-gdwarf -dA" } */
> +
> +typedef short __u16;
> +typedef __u16 u16;
> +__attribute__((btf_type_tag(""))) u16 __softirq_pending;
> +const u16 perm;
> +
> +/* The exact failing pattern is hard to encode with TCL regex machinery,
> + so resort to an indirect way: in the buggy output, const_type points
> + directly to __u16, and only one u16 typedef DIE is emitted (for the
> + btf_type_tag use). In the fixed output, const_type points to a u16
> + typedef, creating a second u16 typedef DIE.
> +/* { dg-final { scan-assembler-times "(DW_AT_name:
> \"u16\"|\"u16..\"\[^\\r\\n\]*DW_AT_name)" 2 } } */
>
--
Richard Biener <[email protected]>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Jochen Jaser, Andrew McDonald, Abhinav Puri; (HRB 36809, AG Nuernberg)
