[Bug exp/28980] New: GDB crashes when using GDB/MI and python pretty printers in some cases

[Bug exp/28980] New: GDB crashes when using GDB/MI and python pretty printers in some cases

[ssbssa at sourceware dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28980

            Bug ID: 28980
           Summary: GDB crashes when using GDB/MI and python pretty
                    printers in some cases
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: normal
          Priority: P2
         Component: exp
          Assignee: unassigned at sourceware dot org
          Reporter: ssbssa at sourceware dot org
  Target Milestone: ---

Originally reported by Jan Vrany on the mailing list:
https://sourceware.org/pipermail/gdb/2022-March/049966.html


TL;DR: 

I'm experiencing GDB crashes with recent GDB which seem to be
caused by commit 5f8ab46. Why is that I do not know (yet). Some 
details below.

Full story:

After updating my every-day-use GDB to commit 

commit c9178f285acf19e066be8367185d52837161b0a2 (HEAD -> master, origin/master)
Author: Alan Modra <amodra@gmail.com>
Date:   Thu Mar 17 20:05:39 2022 +1030

I'm experiencing GDB to crash on assertion failure in value_copy
(value.c:1731).
This is triggered when I single-step through code while having (my, custom)
GDB/MI
frontent displaying local variables with python pretty printers enabled. 

For example: doing this in frontend triggers the assertion failure:

    file gdb/testsuite/outputs/gdb.python/py-prettyprint/py-prettyprint-cxx
    source gdb/testsuite/outputs/gdb.python/py-prettyprint/py-prettyprint.py
    b add_item(container*, int)
    r
    dis 1
    fin
    n
    n (few more times until it crashes)

This is the backtrace when it crashes: 

    /home/jv/Proj...atches/gdb/gdb [stopped]
      Thread  1 [stopped] "gdb"
         0 0x0000556077ADABD0 internal_error (errors.cc:51)
         1 0x00005560776148A7 value_copy (value.c:1731)
         2 0x0000556077360028 gdbpy_get_varobj_pretty_printer
(py-prettyprint.c:655)
         3 0x0000556077620E60 install_default_visualizer (varobj.c:1056)
         4 0x00005560776210E9 install_new_value_visualizer (varobj.c:1127)
         5 0x00005560776218B2 install_new_value (varobj.c:1339)
         6 0x000055607761F6FB varobj_create (varobj.c:378)
         7 0x0000556077245C05 mi_cmd_var_create (mi-cmd-var.c:132)
         8 0x0000556077249F2B mi_command_mi::invoke (mi-cmds.c:58)
         9 0x0000556077264F15 mi_cmd_execute (mi-main.c:2091)
        10 0x000055607726450C captured_mi_execute_command (mi-main.c:1823)
        11 0x000055607726498A mi_execute_command (mi-main.c:1947)
        12 0x000055607724D21B mi_execute_command_wrapper (mi-interp.c:285)
        13 0x000055607724D2A4 mi_execute_command_input_handler
(mi-interp.c:314)
        14 0x00005560770D7149 gdb_readline_no_editing_callback
(event-top.c:878)
        15 0x00005560770D6948 stdin_event_handler (event-top.c:524)
        16 0x0000556077ADB57E gdb_wait_for_event (event-loop.cc:700)
        17 0x0000556077ADB7FB gdb_wait_for_event (event-loop.cc:596)
        18 0x0000556077ADB7FB gdb_do_one_event (event-loop.cc:237)
        19 0x000055607721BA1B start_event_loop (main.c:421)
        20 0x000055607721BB3F captured_command_loop (main.c:481)
        21 0x000055607721D535 captured_main (main.c:1351)
        22 0x000055607721D59B gdb_main (main.c:1366)
        23 0x0000556076DA7E93 main (gdb.c:32)
      Thread  2 [stopped] "gdb worker"
      Thread  3 [stopped] "gdb worker"
      Thread  4 [stopped] "gdb worker"
      Thread  5 [stopped] "gdb worker"


Unfortunately, so far I was unable to reproduce this outside my frontend. I
tried
to simulate what the frontend does (see attached file), but running GDB like 


   gdb -i mi < crash-in-value_copy-reproducer.txt 


does not trigger it (arguably, the frontend issues silly / unnecessary MI
commands, but still
it should not crash GDB - I would think :-). 

If I revert commit


commit 5f8ab46bc6918efb678deb5956c033e466afe301
Author: Simon Marchi <simon.marchi@polymtl.ca>
Date:   Mon Jan 31 15:57:58 2022 -0500

    gdb: constify parameter of value_copy

Everything seems to work just fine for me. I'm not at all familiar with this
part of the GDB code so I do not know whether this change is the real culprit 
or not, let alone to explain why.

I'll try to investigate further when I find more time, but in case someone
brave
enough to read through this post has an idea, I'll appreciate it! 

Thanks, Jan
-------------- next part --------------

file
/home/jv/Projects/gdb/users_jv_patches/gdb/testsuite/outputs/gdb.python/py-prettyprint/py-prettyprint-cxx
source
/home/jv/Projects/gdb/users_jv_patches/gdb/testsuite/outputs/gdb.python/py-prettyprint/py-prettyprint.py
b add_item(container*, int)
r
23-data-list-register-names --thread 1 --frame 0
24-stack-info-depth --thread 1 100
25-data-list-register-values --thread 1 --frame 0 r
26-stack-list-frames --thread 1 0 1
27-stack-list-variables --thread 1 --frame 0 --simple-values
28-thread-info 1
29-var-create --thread 1 --frame 0 - * c
30-var-update --all-values var1
31-var-create --thread 1 --frame 0 - * val
32-var-update --all-values var2
dis 1
fin
33-data-list-register-values --thread 1 --frame 0 r
34-stack-info-depth --thread 1 100
35-stack-list-variables --thread 1 --frame 0 --simple-values
36-stack-list-frames --thread 1 0 0
37-var-create --thread 1 --frame 0 - * ss
38-var-update --all-values var3
39-var-create --thread 1 --frame 0 - * ssa
40-var-update --all-values var4
41-var-create --thread 1 --frame 0 - * arraystruct
42-var-update --all-values var5
43-var-create --thread 1 --frame 0 - * x
44-var-update --all-values var6
45-var-create --thread 1 --frame 0 - * c
46-var-update --all-values var7
47-var-create --thread 1 --frame 0 - * c2
48-var-update --all-values var8
49-var-create --thread 1 --frame 0 - * cstring
50-var-update --all-values var9
51-var-create --thread 1 --frame 0 - * nullstr
52-var-update --all-values var10
53-var-create --thread 1 --frame 0 - * nstype
54-var-update --all-values var11
55-var-create --thread 1 --frame 0 - * nstype2
56-var-update --all-values var12
57-var-create --thread 1 --frame 0 - * me
58-var-update --all-values var13
59-var-create --thread 1 --frame 0 - * ns
60-var-update --all-values var14
61-var-create --thread 1 --frame 0 - * ns2
62-var-update --all-values var15
63-var-create --thread 1 --frame 0 - * estring
64-var-update --all-values var16
65-var-create --thread 1 --frame 0 - * estring2
66-var-update --all-values var17
67-var-create --thread 1 --frame 0 - * estring3
68-var-update --all-values var18
69-exec-next
70-stack-list-variables --thread 1 --frame 0 --simple-values
71-stack-info-depth --thread 1 100
72-stack-list-frames --thread 1 0 0
73-var-create --thread 1 --frame 0 - * ss
74-var-update --all-values var19
75-var-create --thread 1 --frame 0 - * ssa
76-var-update --all-values var20
77-var-create --thread 1 --frame 0 - * arraystruct
78-var-update --all-values var21
79-var-create --thread 1 --frame 0 - * x
80-var-update --all-values var22
81-var-create --thread 1 --frame 0 - * c
82-var-update --all-values var23
83-var-create --thread 1 --frame 0 - * c2
84-var-update --all-values var24
85-var-create --thread 1 --frame 0 - * cstring
86-var-update --all-values var25
87-var-create --thread 1 --frame 0 - * nullstr
88-var-update --all-values var26
89-var-create --thread 1 --frame 0 - * nstype
90-var-update --all-values var27
91-var-create --thread 1 --frame 0 - * nstype2
92-var-update --all-values var28
93-var-create --thread 1 --frame 0 - * me
94-var-update --all-values var29
95-var-create --thread 1 --frame 0 - * ns
96-var-update --all-values var30
97-var-create --thread 1 --frame 0 - * ns2
98-var-update --all-values var31
99-var-create --thread 1 --frame 0 - * estring
100-var-update --all-values var32
101-var-create --thread 1 --frame 0 - * estring2
102-var-update --all-values var33
103-var-create --thread 1 --frame 0 - * estring3
104-var-update --all-values var34
105-exec-next
106-stack-list-variables --thread 1 --frame 0 --simple-values
107-stack-info-depth --thread 1 100
108-stack-list-frames --thread 1 0 0
109-var-create --thread 1 --frame 0 - * ss
110-var-update --all-values var35
111-var-create --thread 1 --frame 0 - * ssa
112-var-update --all-values var36
113-var-create --thread 1 --frame 0 - * arraystruct
114-var-update --all-values var37
115-var-create --thread 1 --frame 0 - * x
116-var-update --all-values var38
117-var-create --thread 1 --frame 0 - * c
118-var-update --all-values var39
119-var-create --thread 1 --frame 0 - * c2
120-var-update --all-values var40
121-var-create --thread 1 --frame 0 - * cstring
122-var-update --all-values var41
123-var-create --thread 1 --frame 0 - * nullstr
124-var-update --all-values var42
125-var-create --thread 1 --frame 0 - * nstype
126-var-update --all-values var43
127-var-create --thread 1 --frame 0 - * nstype2
128-var-update --all-values var44
129-var-create --thread 1 --frame 0 - * me
130-var-update --all-values var45
131-var-create --thread 1 --frame 0 - * ns
132-var-update --all-values var46
133-var-create --thread 1 --frame 0 - * ns2
134-var-update --all-values var47
135-var-create --thread 1 --frame 0 - * estring
136-var-update --all-values var48
137-var-create --thread 1 --frame 0 - * estring2
138-var-update --all-values var49
139-var-create --thread 1 --frame 0 - * estring3
140-var-update --all-values var50
141-exec-next
142-stack-list-variables --thread 1 --frame 0 --simple-values
143-stack-info-depth --thread 1 100
144-stack-list-frames --thread 1 0 0
145-var-create --thread 1 --frame 0 - * ss
146-var-update --all-values var51
147-var-create --thread 1 --frame 0 - * ssa
148-var-update --all-values var52
149-var-create --thread 1 --frame 0 - * arraystruct
150-var-update --all-values var53
151-var-create --thread 1 --frame 0 - * x
152-var-update --all-values var54
153-var-create --thread 1 --frame 0 - * c
154-var-update --all-values var55
155-var-create --thread 1 --frame 0 - * c2
156-var-update --all-values var56
157-var-create --thread 1 --frame 0 - * cstring
158-var-update --all-values var57
159-var-create --thread 1 --frame 0 - * nullstr
160-var-update --all-values var58
161-var-create --thread 1 --frame 0 - * nstype
162-var-update --all-values var59
163-var-create --thread 1 --frame 0 - * nstype2
164-var-update --all-values var60
165-var-create --thread 1 --frame 0 - * me
166-var-update --all-values var61
167-var-create --thread 1 --frame 0 - * ns
168-var-update --all-values var62
169-var-create --thread 1 --frame 0 - * ns2
170-var-update --all-values var63
171-var-create --thread 1 --frame 0 - * estring
172-var-update --all-values var64
173-var-create --thread 1 --frame 0 - * estring2
174-var-update --all-values var65
175-var-create --thread 1 --frame 0 - * estring3
176-var-update --all-values var66
177-exec-next
178-stack-list-variables --thread 1 --frame 0 --simple-values
179-stack-info-depth --thread 1 100
180-stack-list-frames --thread 1 0 0
181-var-create --thread 1 --frame 0 - * ss
182-var-update --all-values var67
183-var-create --thread 1 --frame 0 - * ssa
184-var-update --all-values var68
185-var-create --thread 1 --frame 0 - * arraystruct
186-var-update --all-values var69
187-var-create --thread 1 --frame 0 - * x
188-var-update --all-values var70
189-var-create --thread 1 --frame 0 - * c
190-var-update --all-values var71
191-var-create --thread 1 --frame 0 - * c2
192-var-update --all-values var72
193-var-create --thread 1 --frame 0 - * cstring
194-var-update --all-values var73
195-var-create --thread 1 --frame 0 - * nullstr
196-var-update --all-values var74
197-var-create --thread 1 --frame 0 - * nstype
198-var-update --all-values var75
199-var-create --thread 1 --frame 0 - * nstype2
200-var-update --all-values var76
201-var-create --thread 1 --frame 0 - * me
202-var-update --all-values var77
203-var-create --thread 1 --frame 0 - * ns
204-var-update --all-values var78
205-var-create --thread 1 --frame 0 - * ns2
206-var-update --all-values var79
207-var-create --thread 1 --frame 0 - * estring
208-var-update --all-values var80
209-var-create --thread 1 --frame 0 - * estring2
210-var-update --all-values var81
211-var-create --thread 1 --frame 0 - * estring3
212-var-update --all-values var82
213-exec-next
214-stack-list-variables --thread 1 --frame 0 --simple-values
215-stack-info-depth --thread 1 100
216-stack-list-frames --thread 1 0 0
217-var-create --thread 1 --frame 0 - * ss
218-var-update --all-values var83
219-var-create --thread 1 --frame 0 - * ssa
220-var-update --all-values var84
221-var-create --thread 1 --frame 0 - * arraystruct
222-var-update --all-values var85
223-var-create --thread 1 --frame 0 - * x
224-var-update --all-values var86
225-var-create --thread 1 --frame 0 - * c
226-var-update --all-values var87
227-var-create --thread 1 --frame 0 - * c2
228-var-update --all-values var88
229-var-create --thread 1 --frame 0 - * cstring
230-var-update --all-values var89
231-var-create --thread 1 --frame 0 - * nullstr
232-var-update --all-values var90
233-var-create --thread 1 --frame 0 - * nstype
234-var-update --all-values var91
235-var-create --thread 1 --frame 0 - * nstype2
236-var-update --all-values var92
237-var-create --thread 1 --frame 0 - * me
238-var-update --all-values var93
239-var-create --thread 1 --frame 0 - * ns
240-var-update --all-values var94
241-var-create --thread 1 --frame 0 - * ns2
242-var-update --all-values var95
243-var-create --thread 1 --frame 0 - * estring
244-var-update --all-values var96
245-var-create --thread 1 --frame 0 - * estring2
246-var-update --all-values var97
247-var-create --thread 1 --frame 0 - * estring3
248-var-update --all-values var98
249-exec-next
250-stack-list-variables --thread 1 --frame 0 --simple-values
251-stack-info-depth --thread 1 100
252-stack-list-frames --thread 1 0 0
253-var-create --thread 1 --frame 0 - * ss
254-var-update --all-values var99
255-var-create --thread 1 --frame 0 - * ssa
256-var-update --all-values var100
257-var-create --thread 1 --frame 0 - * arraystruct
258-var-update --all-values var101
259-var-create --thread 1 --frame 0 - * x
260-var-update --all-values var102
261-var-create --thread 1 --frame 0 - * c
262-var-update --all-values var103
263-var-create --thread 1 --frame 0 - * c2
264-var-update --all-values var104
265-var-create --thread 1 --frame 0 - * cstring
266-var-update --all-values var105
267-var-create --thread 1 --frame 0 - * nullstr
268-var-update --all-values var106
269-var-create --thread 1 --frame 0 - * nstype
270-var-update --all-values var107
271-var-create --thread 1 --frame 0 - * nstype2
272-var-update --all-values var108
273-var-create --thread 1 --frame 0 - * me
274-var-update --all-values var109
275-var-create --thread 1 --frame 0 - * ns
276-var-update --all-values var110
277-var-create --thread 1 --frame 0 - * ns2
278-var-update --all-values var111
279-var-create --thread 1 --frame 0 - * estring
280-var-update --all-values var112
281-var-create --thread 1 --frame 0 - * estring2
282-var-update --all-values var113
283-var-create --thread 1 --frame 0 - * estring3
284-var-update --all-values var114
285-exec-next
286-stack-list-variables --thread 1 --frame 0 --simple-values
287-stack-info-depth --thread 1 100
288-stack-list-frames --thread 1 0 0
289-var-create --thread 1 --frame 0 - * ss
290-var-update --all-values var115
291-var-create --thread 1 --frame 0 - * ssa
292-var-update --all-values var116
293-var-create --thread 1 --frame 0 - * arraystruct
294-var-update --all-values var117
295-var-create --thread 1 --frame 0 - * x
296-var-update --all-values var118
297-var-create --thread 1 --frame 0 - * c
298-var-update --all-values var119
299-var-create --thread 1 --frame 0 - * c2
300-var-update --all-values var120
301-var-create --thread 1 --frame 0 - * cstring
302-var-update --all-values var121
303-var-create --thread 1 --frame 0 - * nullstr
304-var-update --all-values var122
305-var-create --thread 1 --frame 0 - * nstype
306-var-update --all-values var123
307-var-create --thread 1 --frame 0 - * nstype2
308-var-update --all-values var124
309-var-create --thread 1 --frame 0 - * me
310-var-update --all-values var125
311-var-create --thread 1 --frame 0 - * ns
312-var-update --all-values var126
313-var-create --thread 1 --frame 0 - * ns2
314-var-update --all-values var127
315-var-create --thread 1 --frame 0 - * estring
316-var-update --all-values var128
317-var-create --thread 1 --frame 0 - * estring2
318-var-update --all-values var129
319-var-create --thread 1 --frame 0 - * estring3
320-var-update --all-values var130
321-exec-next
322-stack-list-variables --thread 1 --frame 0 --simple-values
323-stack-info-depth --thread 1 100
324-stack-list-frames --thread 1 0 0
325-var-create --thread 1 --frame 0 - * ss
326-var-update --all-values var131
327-var-create --thread 1 --frame 0 - * ssa
328-var-update --all-values var132
329-var-create --thread 1 --frame 0 - * arraystruct
330-var-update --all-values var133
331-var-create --thread 1 --frame 0 - * x
332-var-update --all-values var134
333-var-create --thread 1 --frame 0 - * c
334-var-update --all-values var135
335-var-create --thread 1 --frame 0 - * c2
336-var-update --all-values var136
337-var-create --thread 1 --frame 0 - * cstring
338-var-update --all-values var137
339-var-create --thread 1 --frame 0 - * nullstr
340-var-update --all-values var138
341-var-create --thread 1 --frame 0 - * nstype
342-var-update --all-values var139
343-var-create --thread 1 --frame 0 - * nstype2
344-var-update --all-values var140
345-var-create --thread 1 --frame 0 - * me
346-var-update --all-values var141
347-var-create --thread 1 --frame 0 - * ns
348-var-update --all-values var142
349-var-create --thread 1 --frame 0 - * ns2
350-var-update --all-values var143
351-var-create --thread 1 --frame 0 - * estring
352-var-update --all-values var144
353-var-create --thread 1 --frame 0 - * estring2
354-var-update --all-values var145
355-var-create --thread 1 --frame 0 - * estring3
356-var-update --all-values var146
357-exec-next
358-data-list-register-values --thread 1 --frame 0 r
359-gdb-show directories
360-stack-info-depth --thread 1 100
361-stack-list-variables --thread 1 --frame 0 --simple-values
362-stack-list-frames --thread 1 0 1
363-var-create --thread 1 --frame 0 - * main
364-var-update --all-values var147
365-var-create --thread 1 --frame 0 - * argc
366-var-update --all-values var148
367-var-create --thread 1 --frame 0 - * argv
368-var-update --all-values var149
369-var-create --thread 1 --frame 0 - * init
-exec-next

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[Bug exp/28980] GDB crashes when using GDB/MI and python pretty printers in some cases

[ssbssa at sourceware dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28980

Hannes Domani <ssbssa at sourceware dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |12.1

--- Comment #1 from Hannes Domani <ssbssa at sourceware dot org> ---
Can be easily reproduced with python when copying an optimized-out value:

(gdb) py print(gdb.Value(gdb.Value(5).type.optimized_out()))
C:/src/repos/binutils-gdb.git/gdb/value.c:1731: internal-error: value*
value_copy(value*): Assertion `arg->contents != nullptr' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
Quit this debugging session? (y or n) n

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[Bug exp/28980] GDB crashes when using GDB/MI and python pretty printers in some cases

[ssbssa at sourceware dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28980

Hannes Domani <ssbssa at sourceware dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ssbssa at sourceware dot org

-- 
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[Bug exp/28980] GDB crashes when using GDB/MI and python pretty printers in some cases

[jan at vrany dot io]

https://sourceware.org/bugzilla/show_bug.cgi?id=28980

Jan Vrany <jan at vrany dot io> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jan at vrany dot io

--- Comment #2 from Jan Vrany <jan at vrany dot io> ---
*** Bug 28988 has been marked as a duplicate of this bug. ***

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[Bug exp/28980] GDB crashes when using GDB/MI and python pretty printers in some cases

[cvs-commit at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28980

--- Comment #3 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Simon Marchi <simark@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=6d088eb92ee42e05a4fbe797515229cf2acd0d99

commit 6d088eb92ee42e05a4fbe797515229cf2acd0d99
Author: Simon Marchi <simon.marchi@efficios.com>
Date:   Mon Apr 4 17:45:59 2022 -0400

    gdb: don't copy entirely optimized out values in value_copy

    Bug 28980 shows that trying to value_copy an entirely optimized out
    value causes an internal error.  The original bug report involves MI and
    some Python pretty printer, and is quite difficult to reproduce, but
    another easy way to reproduce (that is believed to be equivalent) was
    proposed:

        $ ./gdb -q -nx --data-directory=data-directory -ex "py
print(gdb.Value(gdb.Value(5).type.optimized_out()))"
        /home/smarchi/src/binutils-gdb/gdb/value.c:1731: internal-error:
value_copy: Assertion `arg->contents != nullptr' failed.

    This is caused by 5f8ab46bc691 ("gdb: constify parameter of
    value_copy").  It added an assertion that the contents buffer is
    allocated if the value is not lazy:

      if (!value_lazy (val))
        {
          gdb_assert (arg->contents != nullptr);

    This was based on the comment on value::contents, which suggest that
    this is the case:

      /* Actual contents of the value.  Target byte-order.  NULL or not
         valid if lazy is nonzero.  */
      gdb::unique_xmalloc_ptr<gdb_byte> contents;

    However, it turns out that it can also be nullptr also if the value is
    entirely optimized out, for example on exit of
    allocate_optimized_out_value.  That function creates a lazy value, marks
    the entire value as optimized out, and then clears the lazy flag.  But
    contents remains nullptr.

    This wasn't a problem for value_copy before, because it was calling
    value_contents_all_raw on the input value, which caused contents to be
    allocated before doing the copy.  This means that the input value to
    value_copy did not have its contents allocated on entry, but had it
    allocated on exit.  The result value had it allocated on exit.  And that
    we copied bytes for an entirely optimized out value (i.e. meaningless
    bytes).

    From here I see two choices:

     1. respect the documented invariant that contents is nullptr only and
        only if the value is lazy, which means making
        allocate_optimized_out_value allocate contents
     2. extend the cases where contents can be nullptr to also include
        values that are entirely optimized out (note that you could still
        have some entirely optimized out values that do have contents
        allocated, it depends on how they were created) and adjust
        value_copy accordingly

    Choice #1 is safe, but less efficient: it's not very useful to allocate
    a buffer for an entirely optimized out value.  It's even a bit less
    efficient than what we had initially, because values coming out of
    allocate_optimized_out_value would now always get their contents
    allocated.

    Choice #2 would be more efficient than what we had before: giving an
    optimized out value without allocated contents to value_copy would
    result in an optimized out value without allocated contents (and the
    input value would still be without allocated contents on exit).  But
    it's more risky, since it's difficult to ensure that all users of the
    contents (through the various_contents* accessors) are all fine with
    that new invariant.

    In this patch, I opt for choice #2, since I think it is a better
    direction than choice #1.  #1 would be a pessimization, and if we go
    this way, I doubt that it will ever be revisited, it will just stay that
    way forever.

    Add a selftest to test this.  I initially started to write it as a
    Python test (since the reproducer is in Python), but a selftest is more
    straightforward.

    Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28980
    Change-Id: I6e2f5c0ea804fafa041fcc4345d47064b5900ed7

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[Bug exp/28980] GDB crashes when using GDB/MI and python pretty printers in some cases

[cvs-commit at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28980

--- Comment #4 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
The gdb-12-branch branch has been updated by Simon Marchi
<simark@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=439385561588f12c1df670aaa92af9babd98f2ac

commit 439385561588f12c1df670aaa92af9babd98f2ac
Author: Simon Marchi <simon.marchi@efficios.com>
Date:   Mon Apr 4 17:45:59 2022 -0400

    gdb: don't copy entirely optimized out values in value_copy

    Bug 28980 shows that trying to value_copy an entirely optimized out
    value causes an internal error.  The original bug report involves MI and
    some Python pretty printer, and is quite difficult to reproduce, but
    another easy way to reproduce (that is believed to be equivalent) was
    proposed:

        $ ./gdb -q -nx --data-directory=data-directory -ex "py
print(gdb.Value(gdb.Value(5).type.optimized_out()))"
        /home/smarchi/src/binutils-gdb/gdb/value.c:1731: internal-error:
value_copy: Assertion `arg->contents != nullptr' failed.

    This is caused by 5f8ab46bc691 ("gdb: constify parameter of
    value_copy").  It added an assertion that the contents buffer is
    allocated if the value is not lazy:

      if (!value_lazy (val))
        {
          gdb_assert (arg->contents != nullptr);

    This was based on the comment on value::contents, which suggest that
    this is the case:

      /* Actual contents of the value.  Target byte-order.  NULL or not
         valid if lazy is nonzero.  */
      gdb::unique_xmalloc_ptr<gdb_byte> contents;

    However, it turns out that it can also be nullptr also if the value is
    entirely optimized out, for example on exit of
    allocate_optimized_out_value.  That function creates a lazy value, marks
    the entire value as optimized out, and then clears the lazy flag.  But
    contents remains nullptr.

    This wasn't a problem for value_copy before, because it was calling
    value_contents_all_raw on the input value, which caused contents to be
    allocated before doing the copy.  This means that the input value to
    value_copy did not have its contents allocated on entry, but had it
    allocated on exit.  The result value had it allocated on exit.  And that
    we copied bytes for an entirely optimized out value (i.e. meaningless
    bytes).

    From here I see two choices:

     1. respect the documented invariant that contents is nullptr only and
        only if the value is lazy, which means making
        allocate_optimized_out_value allocate contents
     2. extend the cases where contents can be nullptr to also include
        values that are entirely optimized out (note that you could still
        have some entirely optimized out values that do have contents
        allocated, it depends on how they were created) and adjust
        value_copy accordingly

    Choice #1 is safe, but less efficient: it's not very useful to allocate
    a buffer for an entirely optimized out value.  It's even a bit less
    efficient than what we had initially, because values coming out of
    allocate_optimized_out_value would now always get their contents
    allocated.

    Choice #2 would be more efficient than what we had before: giving an
    optimized out value without allocated contents to value_copy would
    result in an optimized out value without allocated contents (and the
    input value would still be without allocated contents on exit).  But
    it's more risky, since it's difficult to ensure that all users of the
    contents (through the various_contents* accessors) are all fine with
    that new invariant.

    In this patch, I opt for choice #2, since I think it is a better
    direction than choice #1.  #1 would be a pessimization, and if we go
    this way, I doubt that it will ever be revisited, it will just stay that
    way forever.

    Add a selftest to test this.  I initially started to write it as a
    Python test (since the reproducer is in Python), but a selftest is more
    straightforward.

    Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28980
    Change-Id: I6e2f5c0ea804fafa041fcc4345d47064b5900ed7

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[Bug exp/28980] GDB crashes when using GDB/MI and python pretty printers in some cases

[simark at simark dot ca]

https://sourceware.org/bugzilla/show_bug.cgi?id=28980

Simon Marchi <simark at simark dot ca> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |simark at simark dot ca
         Resolution|---                         |FIXED

--- Comment #5 from Simon Marchi <simark at simark dot ca> ---
Pushed to master and gdb-12-branch.

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