[PATCH v2] btf: emit non-representable bitfield as void

[PATCH v2] btf: emit non-representable bitfield as void

[David Faust]

[Changes from v1: use btf_dmd_representable_bitfield_p in
 btf_asm_sou_member, instead of a slightly incorrect equivalent check.]

This patch fixes an issue with mangled BTF that could occur when
a struct type contains a bitfield member which cannot be represented
in BTF.  It is undefined what should happen in such cases, but we can
at least do something reasonable.

Commit

  936dd627cd9 "btf: do not skip members of data type with type id
  BTF_VOID_TYPEID"

made a similar change for un-representable non-bitfield members, but
had an unintended side-effect of mangling BTF for un-representable
bitfields: the struct (or union) would account for the offending
bitfield in its member count but the bitfield member itself was
not emitted, making the member count incorrect.

This change ensures that non-representable bitfield members of struct
and union types are always emitted with BTF_VOID_TYPEID.  This avoids
corrupting the BTF information for the entire struct or union type.

Tested on x86_64-linux-gnu and x86_64-linux-gnu host for
bpf-unknown-none target.

gcc/
	* btfout.cc (btf_asm_sou_member): Always emit non-representable
	bitfield members as having 'void' type.  Refactor slightly.

gcc/testsuite/
	* gcc.dg/debug/btf/btf-bitfields-4.c: Add two new checks.
---
 gcc/btfout.cc                                 | 54 +++++++++----------
 .../gcc.dg/debug/btf/btf-bitfields-4.c        |  2 +
 2 files changed, 28 insertions(+), 28 deletions(-)

diff --git a/gcc/btfout.cc b/gcc/btfout.cc
index ab491f0297f..a1510574a93 100644
--- a/gcc/btfout.cc
+++ b/gcc/btfout.cc
@@ -922,41 +922,39 @@ static void
 btf_asm_sou_member (ctf_container_ref ctfc, ctf_dmdef_t * dmd, unsigned int idx)
 {
   ctf_dtdef_ref ref_type = ctfc->ctfc_types_list[dmd->dmd_type];
+  ctf_id_t base_type = get_btf_id (dmd->dmd_type);
+  uint64_t sou_offset = dmd->dmd_offset;
+
+  dw2_asm_output_data (4, dmd->dmd_name_offset,
+		       "MEMBER '%s' idx=%u",
+		       dmd->dmd_name, idx);
 
   /* Re-encode bitfields to BTF representation.  */
   if (CTF_V2_INFO_KIND (ref_type->dtd_data.ctti_info) == CTF_K_SLICE)
     {
-      ctf_id_t base_type = ref_type->dtd_u.dtu_slice.cts_type;
-      unsigned short word_offset = ref_type->dtd_u.dtu_slice.cts_offset;
-      unsigned short bits = ref_type->dtd_u.dtu_slice.cts_bits;
-      uint64_t sou_offset = dmd->dmd_offset;
-
-      /* Pack the bit offset and bitfield size together.  */
-      sou_offset += word_offset;
-
-      /* If this bitfield cannot be represented, do not output anything.
-	 The parent struct/union 'vlen' field has already been updated.  */
-      if ((bits > 0xff) || (sou_offset > 0xffffff))
-	return;
+      if (btf_dmd_representable_bitfield_p (ctfc, dmd))
+	{
+	  unsigned short word_offset = ref_type->dtd_u.dtu_slice.cts_offset;
+	  unsigned short bits = ref_type->dtd_u.dtu_slice.cts_bits;
 
-      sou_offset &= 0x00ffffff;
-      sou_offset |= ((bits & 0xff) << 24);
+	  /* Pack the bit offset and bitfield size together.  */
+	  sou_offset += word_offset;
+	  sou_offset &= 0x00ffffff;
+	  sou_offset |= ((bits & 0xff) << 24);
 
-      dw2_asm_output_data (4, dmd->dmd_name_offset,
-			   "MEMBER '%s' idx=%u",
-			   dmd->dmd_name, idx);
-      /* Refer to the base type of the slice.  */
-      btf_asm_type_ref ("btm_type", ctfc, get_btf_id (base_type));
-      dw2_asm_output_data (4, sou_offset, "btm_offset");
-    }
-  else
-    {
-      dw2_asm_output_data (4, dmd->dmd_name_offset,
-			   "MEMBER '%s' idx=%u",
-			   dmd->dmd_name, idx);
-      btf_asm_type_ref ("btm_type", ctfc, get_btf_id (dmd->dmd_type));
-      dw2_asm_output_data (4, dmd->dmd_offset, "btm_offset");
+	  /* Refer to the base type of the slice.  */
+	  base_type = get_btf_id (ref_type->dtd_u.dtu_slice.cts_type);
+	}
+      else
+	{
+	  /* Bitfield cannot be represented in BTF.  Emit the member as having
+	     'void' type.  */
+	  base_type = BTF_VOID_TYPEID;
+	}
     }
+
+  btf_asm_type_ref ("btm_type", ctfc, base_type);
+  dw2_asm_output_data (4, sou_offset, "btm_offset");
 }
 
 /* Asm'out an enum constant following a BTF_KIND_ENUM{,64}.  */
diff --git a/gcc/testsuite/gcc.dg/debug/btf/btf-bitfields-4.c b/gcc/testsuite/gcc.dg/debug/btf/btf-bitfields-4.c
index d4a6ef6a1eb..20cdfaa057a 100644
--- a/gcc/testsuite/gcc.dg/debug/btf/btf-bitfields-4.c
+++ b/gcc/testsuite/gcc.dg/debug/btf/btf-bitfields-4.c
@@ -14,6 +14,8 @@
 
 /* Struct with 4 members and no bitfield (kind_flag not set).  */
 /* { dg-final { scan-assembler-times "\[\t \]0x4000004\[\t \]+\[^\n\]*btt_info" 1 } } */
+/* { dg-final { scan-assembler-times " MEMBER" 4 } } */
+/* { dg-final { scan-assembler-times " MEMBER 'unsup' idx=2\[\\r\\n\]+\[^\\r\\n\]*0\[\t \]+\[^\n\]*btm_type: void" 1 } } */
 
 struct bigly
 {
-- 
2.43.0

Re: [PATCH v2] btf: emit non-representable bitfield as void

[Indu Bhagat]

On 4/11/24 14:01, David Faust wrote:
> [Changes from v1: use btf_dmd_representable_bitfield_p in
>   btf_asm_sou_member, instead of a slightly incorrect equivalent check.]
> 
> This patch fixes an issue with mangled BTF that could occur when
> a struct type contains a bitfield member which cannot be represented
> in BTF.  It is undefined what should happen in such cases, but we can
> at least do something reasonable.
> 
> Commit
> 
>    936dd627cd9 "btf: do not skip members of data type with type id
>    BTF_VOID_TYPEID"
> 
> made a similar change for un-representable non-bitfield members, but
> had an unintended side-effect of mangling BTF for un-representable
> bitfields: the struct (or union) would account for the offending
> bitfield in its member count but the bitfield member itself was
> not emitted, making the member count incorrect.
> 
> This change ensures that non-representable bitfield members of struct
> and union types are always emitted with BTF_VOID_TYPEID.  This avoids
> corrupting the BTF information for the entire struct or union type.
> 
> Tested on x86_64-linux-gnu and x86_64-linux-gnu host for
> bpf-unknown-none target.
> 
> gcc/
> 	* btfout.cc (btf_asm_sou_member): Always emit non-representable
> 	bitfield members as having 'void' type.  Refactor slightly.
> 
> gcc/testsuite/
> 	* gcc.dg/debug/btf/btf-bitfields-4.c: Add two new checks.

LGTM.

Thanks

> ---
>   gcc/btfout.cc                                 | 54 +++++++++----------
>   .../gcc.dg/debug/btf/btf-bitfields-4.c        |  2 +
>   2 files changed, 28 insertions(+), 28 deletions(-)
> 
> diff --git a/gcc/btfout.cc b/gcc/btfout.cc
> index ab491f0297f..a1510574a93 100644
> --- a/gcc/btfout.cc
> +++ b/gcc/btfout.cc
> @@ -922,41 +922,39 @@ static void
>   btf_asm_sou_member (ctf_container_ref ctfc, ctf_dmdef_t * dmd, unsigned int idx)
>   {
>     ctf_dtdef_ref ref_type = ctfc->ctfc_types_list[dmd->dmd_type];
> +  ctf_id_t base_type = get_btf_id (dmd->dmd_type);
> +  uint64_t sou_offset = dmd->dmd_offset;
> +
> +  dw2_asm_output_data (4, dmd->dmd_name_offset,
> +		       "MEMBER '%s' idx=%u",
> +		       dmd->dmd_name, idx);
>   
>     /* Re-encode bitfields to BTF representation.  */
>     if (CTF_V2_INFO_KIND (ref_type->dtd_data.ctti_info) == CTF_K_SLICE)
>       {
> -      ctf_id_t base_type = ref_type->dtd_u.dtu_slice.cts_type;
> -      unsigned short word_offset = ref_type->dtd_u.dtu_slice.cts_offset;
> -      unsigned short bits = ref_type->dtd_u.dtu_slice.cts_bits;
> -      uint64_t sou_offset = dmd->dmd_offset;
> -
> -      /* Pack the bit offset and bitfield size together.  */
> -      sou_offset += word_offset;
> -
> -      /* If this bitfield cannot be represented, do not output anything.
> -	 The parent struct/union 'vlen' field has already been updated.  */
> -      if ((bits > 0xff) || (sou_offset > 0xffffff))
> -	return;
> +      if (btf_dmd_representable_bitfield_p (ctfc, dmd))
> +	{
> +	  unsigned short word_offset = ref_type->dtd_u.dtu_slice.cts_offset;
> +	  unsigned short bits = ref_type->dtd_u.dtu_slice.cts_bits;
>   
> -      sou_offset &= 0x00ffffff;
> -      sou_offset |= ((bits & 0xff) << 24);
> +	  /* Pack the bit offset and bitfield size together.  */
> +	  sou_offset += word_offset;
> +	  sou_offset &= 0x00ffffff;
> +	  sou_offset |= ((bits & 0xff) << 24);
>   
> -      dw2_asm_output_data (4, dmd->dmd_name_offset,
> -			   "MEMBER '%s' idx=%u",
> -			   dmd->dmd_name, idx);
> -      /* Refer to the base type of the slice.  */
> -      btf_asm_type_ref ("btm_type", ctfc, get_btf_id (base_type));
> -      dw2_asm_output_data (4, sou_offset, "btm_offset");
> -    }
> -  else
> -    {
> -      dw2_asm_output_data (4, dmd->dmd_name_offset,
> -			   "MEMBER '%s' idx=%u",
> -			   dmd->dmd_name, idx);
> -      btf_asm_type_ref ("btm_type", ctfc, get_btf_id (dmd->dmd_type));
> -      dw2_asm_output_data (4, dmd->dmd_offset, "btm_offset");
> +	  /* Refer to the base type of the slice.  */
> +	  base_type = get_btf_id (ref_type->dtd_u.dtu_slice.cts_type);
> +	}
> +      else
> +	{
> +	  /* Bitfield cannot be represented in BTF.  Emit the member as having
> +	     'void' type.  */
> +	  base_type = BTF_VOID_TYPEID;
> +	}
>       }
> +
> +  btf_asm_type_ref ("btm_type", ctfc, base_type);
> +  dw2_asm_output_data (4, sou_offset, "btm_offset");
>   }
>   
>   /* Asm'out an enum constant following a BTF_KIND_ENUM{,64}.  */
> diff --git a/gcc/testsuite/gcc.dg/debug/btf/btf-bitfields-4.c b/gcc/testsuite/gcc.dg/debug/btf/btf-bitfields-4.c
> index d4a6ef6a1eb..20cdfaa057a 100644
> --- a/gcc/testsuite/gcc.dg/debug/btf/btf-bitfields-4.c
> +++ b/gcc/testsuite/gcc.dg/debug/btf/btf-bitfields-4.c
> @@ -14,6 +14,8 @@
>   
>   /* Struct with 4 members and no bitfield (kind_flag not set).  */
>   /* { dg-final { scan-assembler-times "\[\t \]0x4000004\[\t \]+\[^\n\]*btt_info" 1 } } */
> +/* { dg-final { scan-assembler-times " MEMBER" 4 } } */
> +/* { dg-final { scan-assembler-times " MEMBER 'unsup' idx=2\[\\r\\n\]+\[^\\r\\n\]*0\[\t \]+\[^\n\]*btm_type: void" 1 } } */
>   
>   struct bigly
>   {