json_parse_number broken by compiler optimization

Tue Mar 30 20:34:54 EEST 2021

On Tue, Mar 30, 2021 at 17:53:27 +0200, Christian Ehrhardt wrote:
> Hi,
> the recent Ubuntu (re)builds uncovered an issue with dovecot 1:2.3.13+dfsg1-1
> build log: https://launchpadlibrarian.net/529849650/buildlog_ubuntu-hirsute-amd64.dovecot_1%3A2.3.13+dfsg1-1build1_BUILDING.txt.gz
> A coworker tried 2.3.14 but got the same result.
> 
> What fails is the json_parser build time test like:
>   test-json-parser.c:161: Assert(#25) failed:
> null_strcmp(json_output[pos].value, value) == 0
> 
> I was looking into that a bit more and what I found is that it is
> dependent on the new toolchain
> of gcc 10.2.0-1.

FWIW, I managed to reproduce it on FreeBSD with gcc 11, so the good news for
you is that it isn't Ubuntu specific :)

I'll debug further.

Thanks again for the report,

Jeff.

> 
> Not all calls to json_parse_* fail, e.g. the first one looks all good and passes
> I was iterating the tests using a report function defined like
> 
> (gdb) define repcon
> >c
> >p pos
> >p json_output[pos].type
> >p type
> >p json_output[pos].value
> >p value
> >call null_strcmp(json_output[pos].value, value)
> >end
> 
> The first one to be bad was:
> Breakpoint 2, test_json_parser_success (full_size=<optimized out>) at
> test-json-parser.c:161
> 161 test_assert_idx(null_strcmp(json_output[pos].value, value) == 0, pos);
> $84 = 25
> $85 = JSON_TYPE_NUMBER
> $86 = JSON_TYPE_NUMBER
> $87 = 0x555555633b25 "-12.456"
> $88 = 0x555555693110 ""
> $89 = 45
> 
> Earlier and later parsing was happy, for example
> 
> Breakpoint 2, test_json_parser_success (full_size=<optimized out>) at
> test-json-parser.c:161
> 161 test_assert_idx(null_strcmp(json_output[pos].value, value) == 0, pos);
> $90 = 27
> $91 = JSON_TYPE_NUMBER
> $92 = JSON_TYPE_NUMBER
> $93 = 0x555555633b32 "12.456e9"
> $94 = 0x555555693110 "12.456e9"
> $95 = 0
> (gdb)
> 
> 
> We have two things we compare here.
> 1. json_output[] which is a static define and for this value is
>    67 »···{ JSON_TYPE_NUMBER, "-12.456" },
> 2. the return value that json_parse_next returns.
>    25 »···"  \"sub2\":-12.456,\n"
>   148 »···»···»···»···ret = json_parse_next(parser, &type, &value);
> 
> I tried a non negative number and got success which is suspicious
> 
> Breakpoint 5, test_json_parser_success (full_size=<optimized out>) at
> test-json-parser.c:164
> 164 test_assert_idx(null_strcmp(json_output[pos].value, value) == 0, pos);
> $122 = 25
> $123 = JSON_TYPE_NUMBER
> $124 = JSON_TYPE_NUMBER
> $125 = 0x555555633b2c "12.456"
> $126 = 0x555555693110 "12.456"
> $127 = 0
> (gdb)
> 
> 
> Also the return value otherwise LGTM, it is recognized as a number:
> 540 } else if ((ret = json_parse_number(parser, value_r)) >= 0) {
> (gdb) n
> 541 *type_r = JSON_TYPE_NUMBER;
> 
> So parsing worked one might think, but the value was not set.
> 
> (gdb) p *value_r
> $833 = 0x555555693110 ""
> 
> It turns out json_parse_number is too optimized.
> Too optimized for debugging further into json_parse_number.
> But also optimization is causing the issue.
> With -O0 or -O1 it works fine, even putting the lower optimization
> level just around json_parse_number it is enough to avoid the issue.
> 
> 
> Not a fix but "avoidance" for now:
> 
> --- a/src/lib/json-parser.c
> +++ b/src/lib/json-parser.c
> @@ -386,6 +386,8 @@ static int json_parse_int(struct json_pa
>   return 1;
>  }
> 
> +#pragma GCC push_options
> +#pragma GCC optimize ("-O0")
>  static int json_parse_number(struct json_parser *parser, const char **value_r)
>  {
>   int ret;
> @@ -415,6 +417,7 @@ static int json_parse_number(struct json
>   *value_r = str_c(parser->value);
>   return 1;
>  }
> +#pragma GCC pop_options
> 
>  static int json_parse_atom(struct json_parser *parser, const char *atom)
>  {
> 
> 
> I was trying to see what happened and tracked each str_append_c
> that happened inside json_parse_number.
> A breakpoint on str_append_c and backtraces showed that we clearly do the
> "right" parsing. From the backtraces I've seen it does (just from positions):
> 
> For example
> (gdb) bt
> #0  str_append_c (chr=<optimized out>, str=0x5555556930d0) at str.h:44
> #1  json_parse_digits (parser=<optimized out>) at json-parser.c:366
> => that is a digit
> 
> It does "- digit digit dot digit digit digit"
> 
> This perfectly matches the input value, but still the result stays empty:
> (gdb) p (char*)(str->data)
> $738 = 0x555555693110 ""
> 
> In comparison - when parsing the latter 12.456e9 which works I see two
> odd things:
> 1. the value isn't cleared despite
>     str_truncate(parser->value, 0);
>    at the beginning of json_parse_number
> 2. the value (this time) is read exactly as I assumed
> 
> Breakpoint 17, json_parse_number (value_r=0x7fffffffe1a0,
> parser=0x555555692ee0) at json-parser.c:394
> 394 str_truncate(parser->value, 0);
> Breakpoint 18, str_append_c (chr=<optimized out>, str=0x5555556930d0)
> at str.h:44
> 44 buffer_append_c(str, chr);
> (gdb) p (char*)(str->data)
> $741 = 0x555555693110 "sub3"
> 
> ^^ not cleared, still has "sub3" even later on
> 
> Breakpoint 18, str_append_c (chr=<optimized out>, str=0x5555556930d0)
> at str.h:44
> 44 buffer_append_c(str, chr);
> (gdb) p (char*)(str->data)
> $742 = 0x555555693110 "1ub3"
> 
> ^^ reading left to right one char at a time "over the old value",
> which didn't update the data.
> 
> So it seems in json_parse_number the ordering of
> a) str_truncate(parser->value, 0);
> b) the parsing itself
> is messed up by optimization.
> 
> We already know from the above that reducing just this function to -O1 or less
> avoids the issue. But what is it really - I don't know?
> 
> Maybe you have a better idea what is going on, the issue should be
> reproducible when building
> dovecot with the toolchain present on the latest Ubuntu being 21.04
> (Hirsute) right now.
> 
> --
> Christian Ehrhardt
> Staff Engineer, Ubuntu Server
> Canonical Ltd