Fix repeated binary segment bug #10588
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The following snippet reads garbage memory and even often triggers
segfaults. It barely ever happens with small Ns, but for around larger
than 40 it is almost guaranteed to segfault in my machine.
```erl
generate_max_length_names(N) ->
Label = <<<<$a>> || _ <- lists:seq(1, N)>>,
<<Label/binary, Label/binary, Label/binary>>.
```
I found this while trying to write some tests for bencharking dns packet
parsing logic. It did not trigger with only two labels, but with three
it is enough. These labels trigger or fail to trigger too:
```erl
BadLabel = <<<<$a>> || <<_>> <= <<1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1>> >>,
GoodLabel = <<1, 1, 1, 1, 1,1, 1, 1, 1, 1,1, 1, 1, 1, 1,1, 1, 1, 1, 1,1, 1, 1, 1, 1>>,
GoodLabel = iolist_to_binary(lists:duplicate(63, <<$a>>)),
```
My findings:
When the same binary variable is used multiple times in a single
bs_create_bin instruction (e.g., `<<Label/binary, Label/binary,
Label/binary>>`), the `private_append` optimization was incorrectly
applied. This caused memory corruption because:
1. The first segment uses `private_append` to destructively extend
the binary
2. The `ErlSubBits` structure is modified to reflect the new size
3. Subsequent segments read this modified size, not the original
4. The runtime copies more bytes than the original contained,
reading uninitialized/garbage memory
The fix adds a check to ensure that before setting `first_fragment_dies`
to true (which enables `private_append`), we verify the first fragment
variable doesn't appear in any other segment of the instruction.
Contributor
CT Test Results 2 files 333 suites 8m 29s ⏱️ Results for commit 466bd36. ♻️ This comment has been updated with latest results. To speed up review, make sure that you have read Contributing to Erlang/OTP and that all checks pass. See the TESTING and DEVELOPMENT HowTo guides for details about how to run test locally. Artifacts
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The following snippet reads garbage memory and even often triggers segfaults. It barely ever happens with small Ns, but for around larger than 40 it is almost guaranteed to segfault in my machine.
I found this while trying to write some tests for bencharking dns packet parsing logic. It did not trigger with only two labels, but with three it is enough. These labels trigger or fail to trigger too:
My findings:
When the same binary variable is used multiple times in a single bs_create_bin instruction (e.g.,
<<Label/binary, Label/binary, Label/binary>>), theprivate_appendoptimization was incorrectly applied. This caused memory corruption because:private_appendto destructively extend the binaryErlSubBitsstructure is modified to reflect the new sizeThe fix adds a check to ensure that before setting
first_fragment_diesto true (which enablesprivate_append), we verify the first fragment variable doesn't appear in any other segment of the instruction.