MDEV-41179 MAX_KEY_PARTS ranges in select causing SEGV - #5782
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DaveGosselin-MariaDB
September 29, 2026 00:02
When a select containing 32 ranges is made on a table containing a
compound key with 32 parts, the range optimizer can run off the end
of a stack variable, invalidly overwriting subsequent stack variables.
In the struct st_sel_arg_range_seq, we have an array
RANGE_SEQ_ENTRY stack[MAX_REF_PARTS];
MAX_REF_PARTS is 32.
check_quick_select
/ sel_arg_range_seq_init
initialises stack[0] as NOT a key part
/ sel_arg_range_seq_next
iterates through the key parts, adding key part n to stack[n+1]
key part #32 gets referenced by step_down_to(), setting seq->i off the
end of the array.
The above change has exposed an issue with key length calculation
on MS Windows. The field is a ulong and with all 32 key parts
wanted, we calculate this as (1UL << 32) - 1. The first part of
the calculation overflows the field. The result is undefined in C++.
Fix: change key_part_map to 64 bits long, so it is the same on Linux
and MS Windows.
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When a select containing 32 ranges is made on a table containing a compound key with 32 parts, the range optimizer can run off the end of a stack variable, invalidly overwriting subsequent stack variables.
In the struct st_sel_arg_range_seq, we have an array
RANGE_SEQ_ENTRY stack[MAX_REF_PARTS];
MAX_REF_PARTS is 32.
check_quick_select
/ sel_arg_range_seq_init
initialises stack[0] as NOT a key part
/ sel_arg_range_seq_next
iterates through the key parts, adding key part n to stack[n+1]
key part #32 gets referenced by step_down_to(), setting seq->i off the end of the array.
The above change has exposed an issue with key length calculation on MS Windows. The field is a ulong and with all 32 key parts wanted, we calculate this as (1UL << 32) - 1. The first part of the calculation overflows the field. The result is undefined in C++.
Fix: change key_part_map to 64 bits long, so it is the same on Linux
and MS Windows.