Re: json_parse_number broken by compiler optimization

On 30/03/2021 18:53 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.dovec... 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.

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

Thanks! We'll look into this. Aki