Re: [PATCH 2/6] gdb: handle incorrect CU size in .gdb_index

[Simon Marchi <simark@simark.ca>]

On 10/18/26 4:50 PM, Andrew Burgess wrote:
> The CU list within the .gdb_index contains the offset of a CU and the
> length of the CU.
> 
> The length field is not needed when parsing the .gdb_index, it is
> currently only used within GDB to pre-fill the length field of the
> dwarf2_per_cu object.  This pre-filled length is then validated in
> dwarf2_per_cu::set_length when the real CU header is read from the
> .debug_info.
> 
> Unfortunately, this validation is currently done using a gdb_assert,
> which is the wrong approach.  Both the .gdb_index and the CU header
> are untrusted external input.  Either of them (or both of them) might
> be wrong, and though they should have the same value, this is not
> guaranteed.
> 
> I cannot just replace the gdb_assert in dwarf2_per_cu::set_length with
> a call to complaint, there are other paths where the length is
> pre-filled from the actual CU header, in this case there should be no
> possibility of a mismatch, and so gdb_assert is the correct approach.
> 
> I could add a new member variable to dwarf2_per_cu that tracks if the
> pre-filled length came from the real CU header or from the .gdb_index,
> then we could switch on this to either call gdb_assert or complaint.
> 
> But this seemed excessive when complaints are usually turned off by
> default.
> 
> Instead, I propose we just ignore the CU length from the .gdb_index,
> and pass 0 instead when initially creating the dwarf2_per_cu object.
> A length of 0 indicates that the CU length is unknown.  When GDB reads
> the actual CU header it will then set the length at this point.
> 
> The CU length in the .gdb_index is no longer being used, but I am not
> proposing that we change the .gdb_index format, though we could in
> theory bump the index version number and remove the CU length, saving
> a little space.  However, changing the index format means more work
> for downstream tools that read or write .gdb_index sections, so
> avoiding that churn seems best to me.

I think this is ok, dwarf2_find_containing_unit (one of the only
things that uses a unit's length) is ready for this.

Simon