Re: [PATCH v2] gdb: call check_typedef at beginning of dwarf_expr_context::fetch_result

[Simon Marchi via Gdb-patches <gdb-patches@sourceware.org>]

On 2022-07-27 21:34, Simon Marchi wrote:
> From: Simon Marchi <simon.marchi@polymtl.ca>
> 
> Bug 29374 shows this crash:
> 
>     $ ./gdb -nx --data-directory=data-directory -q -batch -ex "catch throw" -ex r -ex bt a.out
>     ...
>     /home/simark/src/binutils-gdb/gdb/../gdbsupport/array-view.h:217: internal-error: copy: Assertion `dest.size () == src.size ()' failed.
> 
> The backtrace is:
> 
>     #0  internal_error (file=0x5555606504c0 "/home/simark/src/binutils-gdb/gdb/../gdbsupport/array-view.h", line=217, fmt=0x55556064b700 "%s: Assertion `%s' failed.") at /home/simark/src/binutils-gdb/gdbsupport/errors.cc:51
>     #1  0x000055555d41c0bb in gdb::copy<unsigned char const, unsigned char> (src=..., dest=...) at /home/simark/src/binutils-gdb/gdb/../gdbsupport/array-view.h:217
>     #2  0x000055555deef28c in dwarf_expr_context::fetch_result (this=0x7fffffffb830, type=0x621007a86830, subobj_type=0x621007a86830, subobj_offset=0, as_lval=false) at /home/simark/src/binutils-gdb/gdb/dwarf2/expr.c:1040
>     #3  0x000055555def0015 in dwarf_expr_context::evaluate (this=0x7fffffffb830, addr=0x62f00004313e "0", len=1, as_lval=false, per_cu=0x60b000069550, frame=0x621007c9e910, addr_info=0x0, type=0x621007a86830, subobj_type=0x621007a86830, subobj_offset=0) at /home/simark/src/binutils-gdb/gdb/dwarf2/expr.c:1091
>     #4  0x000055555e084327 in dwarf2_evaluate_loc_desc_full (type=0x621007a86830, frame=0x621007c9e910, data=0x62f00004313e "0", size=1, per_cu=0x60b000069550, per_objfile=0x613000006080, subobj_type=0x621007a86830, subobj_byte_offset=0, as_lval=false) at /home/simark/src/binutils-gdb/gdb/dwarf2/loc.c:1485
>     #5  0x000055555e0849e2 in dwarf2_evaluate_loc_desc (type=0x621007a86830, frame=0x621007c9e910, data=0x62f00004313e "0", size=1, per_cu=0x60b000069550, per_objfile=0x613000006080, as_lval=false) at /home/simark/src/binutils-gdb/gdb/dwarf2/loc.c:1529
>     #6  0x000055555e0828c6 in dwarf_entry_parameter_to_value (parameter=0x621007a96e58, deref_size=0x0, type=0x621007a86830, caller_frame=0x621007c9e910, per_cu=0x60b000069550, per_objfile=0x613000006080) at /home/simark/src/binutils-gdb/gdb/dwarf2/loc.c:1235
>     #7  0x000055555e082f55 in value_of_dwarf_reg_entry (type=0x621007a86890, frame=0x621007acc510, kind=CALL_SITE_PARAMETER_DWARF_REG, kind_u=...) at /home/simark/src/binutils-gdb/gdb/dwarf2/loc.c:1332
>     #8  0x000055555e083449 in value_of_dwarf_block_entry (type=0x621007a86890, frame=0x621007acc510, block=0x61e000033568 "T\004\205\001\240\004\004\243\001T\237\004\240\004\261\004\001T\004\261\004\304\005\004\243\001T\237\004\304\005\310\005\001T\004\310\005\311\005\004\243\001T\237", block_len=1) at /home/simark/src/binutils-gdb/gdb/dwarf2/loc.c:1365
>     #9  0x000055555e094d40 in loclist_read_variable_at_entry (symbol=0x621007a99bd0, frame=0x621007acc510) at /home/simark/src/binutils-gdb/gdb/dwarf2/loc.c:3889
>     #10 0x000055555f5192e0 in read_frame_arg (fp_opts=..., sym=0x621007a99bd0, frame=0x621007acc510, argp=0x7fffffffbf20, entryargp=0x7fffffffbf60) at /home/simark/src/binutils-gdb/gdb/stack.c:559
>     #11 0x000055555f51c352 in print_frame_args (fp_opts=..., func=0x621007a99ad0, frame=0x621007acc510, num=-1, stream=0x6030000bad90) at /home/simark/src/binutils-gdb/gdb/stack.c:887
>     #12 0x000055555f521919 in print_frame (fp_opts=..., frame=0x621007acc510, print_level=1, print_what=LOCATION, print_args=1, sal=...) at /home/simark/src/binutils-gdb/gdb/stack.c:1390
>     #13 0x000055555f51f22e in print_frame_info (fp_opts=..., frame=0x621007acc510, print_level=1, print_what=LOCATION, print_args=1, set_current_sal=0) at /home/simark/src/binutils-gdb/gdb/stack.c:1116
>     #14 0x000055555f526c6d in backtrace_command_1 (fp_opts=..., bt_opts=..., count_exp=0x0, from_tty=0) at /home/simark/src/binutils-gdb/gdb/stack.c:2079
>     #15 0x000055555f527ae5 in backtrace_command (arg=0x0, from_tty=0) at /home/simark/src/binutils-gdb/gdb/stack.c:2198
> 
> The problem is that the type that gets passed down to
> dwarf_expr_context::fetch_result (the type of a variable of which we're
> trying to read the entry value) is a typedef whose size has never been
> computed yet (check_typedef has never been called on it).  As we get in
> the DWARF_VALUE_STACK case (line 1028 of dwarf2/expr.c), the `len`
> variable is therefore set to 0, instead of the actual type length.  We
> then call allocate_value on subobj_type, which does call check_typedef,
> so the length of the typedef gets filled in at that point.  We end up
> passing to the copy function a source array view of length 0 and a
> target array view of length 4, and the assertion fails.
> 
> Fix this by calling check_typedef on both type and subobj_type at the
> beginning of fetch_result.
> 
> I tried writing a test for this using the DWARF assembler, but I haven't
> succeeded.  It's possible that we need to get into this specific code
> path (value_of_dwarf_reg_entry and all) to manage to get to
> dwarf_expr_context::fetch_result with a typedef type that has never been
> resolved.  In all my attempts, the typedef would always be resolved
> already, so the bug wouldn't show up.
> 
> As a fallback, I made a gdb.dwarf2 test with a compiler-generated .S
> file.  I don't particularly like those, but I think it's better than no
> test.  The .cpp source code is the smallest reproducer I am able to make
> from the reproducer given in the bug (thanks to Pedro for suggestions
> on how to minimize it further than I had).
> 
> Change-Id: I182733ad08e34df40d8bcc47af72c482fabf4900
> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29374

Sorry, I forgot that I had already sent this patch.  So this is effectively the v2
of this one (I fixed the subject accordingly):

  https://sourceware.org/pipermail/gdb-patches/2022-July/191035.html

This new patch has a test.

Simon