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authorAndrew Burgess <aburgess@redhat.com>2021-12-07 14:01:23 +0000
committerAndrew Burgess <aburgess@redhat.com>2022-02-02 16:27:36 +0000
commit3c5fcec6dccb0e598d1e64640e55d50ed3ddedb6 (patch)
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gdb: handle calls to list command passing only a linespec condition
In PR cli/28665, it was reported that GDB would crash when given a command like: (gdb) list task 123 The problem here is that in cli/cli-cmd.c:list_command, the string 'task 123' is passed to string_to_event_location in find a location specification. However, this location parsing understands about breakpoint conditions, and so, will stop parsing when it sees something that looks like a condition, in this case, the 'task 123' looks like a breakpoint condition. As a result, the location we get back from string_to_event_location has no actual location specification attached to it. The actual call path is: list_command string_to_event_location string_to_event_location_basic new_linespec_location In new_linespec_location we call linespec_lex_to_end, which looks at 'task 123' and decides that there's nothing there that describes a location. As such, in new_linespec_location, the spec_string field of the location is left as nullptr. Back in list_command we then call decode_line_1, which calls event_location_to_sals, which calls parse_linespec, which takes the spec_string we found earlier, and tries to converts this into a list of sals. However, parse_linespec is not intended to be passed a nullptr, for example, calling is_ada_operator will try to access through the nullptr, causing undefined behaviour. But there are other cases within parse_linespec which don't expect to see a nullptr. When looking at how to fix this issue, I first considered having linespec_lex_to_end detect the problem. That function understands when the first thing in the linespec is a condition keyword, and so, could throw an error saying something like: "no linespec before condition keyword", however, this is not going to work, at least, not without additional changes to GDB, it is valid to place a breakpoint like: (gdb) break task 123 This will place a breakpoint at the current location with the condition 'task 123', and changing linespec_lex_to_end breaks this behaviour. So, next, I considered what would happen if I added a condition to an otherwise valid list command, this is what I see: (gdb) list file.c:1 task 123 Junk at end of line specification. (gdb) So, then I wondered, could we just pull the "Junk" detection forward, so that we throw the error earlier, before we call decode_line_1? It turns out that yes we can. Well, sort of. It is simpler, I think, to add a separate check into the list_command function, after calling string_to_event_location, but before calling decode_line_1. We know when we call string_to_event_location that the string in question is not empty, so, after calling string_to_event_location, if non of the string has been consumed, then the content of the string must be junk - it clearly doesn't look like a location specification. I've reused the same "Junk at end of line specification." error for consistency, and added a few tests to cover this issue. While the first version of this patch was on the mailing list, a second bug PR gdb/28797 was raised. This was for a very similar issue, but this time the problem command was: (gdb) list ,, Here the list command understands about the first comma, list can have two arguments separated by a comma, and the first argument can be missing. So we end up trying to parse the second command "," as a linespec. However, in linespec_lex_to_end, we will stop parsing a linespec at a comma, so, in the above case we end up with an empty linespec (between the two commas), and, like above, this results in the spec_string being nullptr. As with the previous case, I've resolved this issue by adding an extra check for junk at the end of the line - after parsing (or failing to parse) the nothing between the two commas, we still have the "," left at the end of the list command line - when we see this we can throw the same "junk at the end of the line" error, and all is good. I've added tests for this case too. Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28665 Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28797
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