Re: [PATCH dwarves] btf_encoder: Fix a dwarf type DW_ATE_unsigned_1024 to btf encoding issue

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On 4/26/23 11:09 AM, Nick Desaulniers wrote:
On Tue, Apr 25, 2023 at 10:50 PM Yonghong Song <yhs@xxxxxx> wrote:

Nick Desaulniers reported an issue ([1]) where an 128-byte sized type
(DW_ATE_unsigned_1024) cannot be encoded into BTF with failure message
likes below:
   $ pahole -J reduced.o
   [2] INT DW_ATE_unsigned_1024 Error emitting BTF type
   Encountered error while encoding BTF.
See [1] for how to reproduce the issue.

The failure is due to currently BTF int type only supports upto 16
bytes (__int128) and in this case the dwarf int type is 128-byte.

The DW_ATE_unsigned_1024 is not a normal type for variable/func
declaration etc. It is used in DW_AT_location. There are two
ways to resolve this issue.
   (1). If btf encoding is expected, remove all dwarf int types
        where btf encoding will failure, e.g., non-power-of-2
        bytes, or greater than 16 bytes.
   (2). do a sanitization in btf_encoder ([2]).

This patch uses method (2) since it is a simple fix in btf_encoder.
I checked my local built vmlinux with latest
bpf-next. There is only one instance of DW_ATE_unsigned_24 (used in
DW_AT_location) so I expect irregular int types should be very rare.

   [1] https://github.com/libbpf/libbpf/pull/680
   [2] commit 7d8e829f636f ("btf_encoder: Sanitize non-regular int base type")

Signed-off-by: Yonghong Song <yhs@xxxxxx>

Thanks, this fixed the above reported error for me.  My report is just
forwarded from Satya.

Reported-by: Satya Durga Srinivasu Prabhala <quic_satyap@xxxxxxxxxxx>
Tested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>

I don't know if that change has other implications for unusual byte sizes.

We might need to consider at some point waiting to validate
DW_TAG_base_type until we know that they're not used outside of
DW_AT_location expressions.

David Blaikie confirmed that indeed special types like DW_ATE_unsigned_1024 is indeed generated for DW_AT_location.
See https://github.com/libbpf/libbpf/pull/680 for details.


---
  btf_encoder.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/btf_encoder.c b/btf_encoder.c
index 65f6e71..1aa0ad0 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -394,7 +394,7 @@ static int32_t btf_encoder__add_base_type(struct btf_encoder *encoder, const str
          * these non-regular int types to avoid libbpf/kernel complaints.
          */
         byte_sz = BITS_ROUNDUP_BYTES(bt->bit_size);
-       if (!byte_sz || (byte_sz & (byte_sz - 1))) {
+       if (!byte_sz || (byte_sz & (byte_sz - 1)) || byte_sz > 16) {
                 name = "__SANITIZED_FAKE_INT__";
                 byte_sz = 4;
         }
--
2.34.1






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