https://sourceware.org/bugzilla/show_bug.cgi?id=34857
Bug ID: 34857
Summary: GNU ld dereferences a null output BFD in a
SEGMENT_START fill expression
Product: binutils
Version: 2.47
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: ld
Assignee: unassigned at sourceware dot org
Reporter: basavesh.shivakumar at wsu dot edu
Target Milestone: ---
Created attachment 17177
--> https://sourceware.org/bugzilla/attachment.cgi?id=17177&action=edit
repro
I'm reporting this bug on behalf of my team: Ananta Srikar Puranam, Emily West,
Dr. Swarn Priya, Dr. Peng Gao and Dr. Basavesh Ammanaghatta Shivakumar.
Summary
=======
A syntactically valid x86-64 linker script that uses SEGMENT_START as an
output-section fill expression makes current and official 2.47 GNU ld evaluate
bfd_get_flavour(link_info.output_bfd) before output_bfd exists, causing
deterministic null-pointer UB; a literal-fill control links cleanly and system
GNU ld 2.42 predating commit 6286d4a21187 accepts both.
Reproduction
============
1. Download and extract
`bfd-segment-start-fill-null-pointer-ubsan-reproducer.tar.gz`.
2. Run the following command with an ld.bfd or all-target `ld-new` binary that
supports the emulation used by the reproducer:
LD_BFD=/path/to/ld-new sh repro.sh
The archive contains the fixed input files, independent controls, helper
scripts needed by `repro.sh`, and their checksums. The script uses a temporary
directory and does not modify the source tree.
Observed result
===============
A syntactically valid x86-64 linker script that uses SEGMENT_START as an
output-section fill expression makes current and official 2.47 GNU ld evaluate
bfd_get_flavour(link_info.output_bfd) before output_bfd exists, causing
deterministic null-pointer UB; a literal-fill control links cleanly and system
GNU ld 2.42 predating commit 6286d4a21187 accepts both.
Expected result
===============
GNU ld should either produce the correct output or reject the input with a
normal diagnostic. It should not crash, hang, execute undefined behavior, read
outside an input-derived allocation, or produce the described malformed
output.
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