[Bug gas/30857] New: Use of uninitialized memory

[jacob at jacob dot remcomp.fr]

https://sourceware.org/bugzilla/show_bug.cgi?id=30857

            Bug ID: 30857
           Summary: Use of uninitialized memory
           Product: binutils
           Version: 2.15
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: gas
          Assignee: unassigned at sourceware dot org
          Reporter: jacob at jacob dot remcomp.fr
  Target Milestone: ---

FUNCTION: riscv_ip_hardcode
FILE: gas/config/tc-riscv.c LINE: 3682
Problem: Usage of uninitialized memory.
Variable: Local variable of type "riscv_opcode *" insn.

Description:
This variable is initialized with a call to XNEW(struct riscv_opcode);
3721:   insn = XNEW(struct riscv_opcode);
All fields of this structure are garbage since we called malloc.
The next line initializes ONE of those fields:
3722:   insn->match = values[num - 1];
Then, a call to "create_insn" is done:
        create_insn(ip,insn);
The function "create_insn" initializes its left argument with the values of
its right argument. In this case however, it will "initialize" its left
argument with a structure that contains mostly garbage since only ONE field
has been really initialized!

There is only ONE place where riscv_ip_hardcode is called: in function
s_riscv_insn. After the call, s_riscv_insn assumes that insn has been
correctly initialized and makes:
4868:   gas_assert(insn.insn_mo->pinfo != INSN_MACRO);
without realizing that insn.insn_mo->pinfo is a garbage value.

Analysis: Garbage values are unlike to be 0xffffffff, the value of 
INSN_MACRO, so in most cases this inequality will be true, and the code
continues to run as if nothing would be wrong. In some cases the code
will fail with an "assertion failed" message. Since this bug is not
reproducible... any bug reports will be discarded.

HOW TO FIX:
1) Intead of calling XNEW call XCNEW that calls calloc instead of malloc.
  This will ensure that the inequality will fail.
2) Initialize all values to sensible values. This is much more difficult and
  involves much more effort, probably for nothing since those values aren't
  used.

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