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Re: [Qemu-devel] [PATCH 0/6]: QMP: Fix issues in parser/lexer
From: |
Anthony Liguori |
Subject: |
Re: [Qemu-devel] [PATCH 0/6]: QMP: Fix issues in parser/lexer |
Date: |
Wed, 19 May 2010 16:43:08 -0500 |
User-agent: |
Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.5) Gecko/20091209 Fedora/3.0-4.fc12 Lightning/1.0pre Thunderbird/3.0 |
On 05/19/2010 04:15 PM, Luiz Capitulino wrote:
Hi Anthony,
While investigating a QMP bug reported by a user, I've found a few issues
in our parser/lexer.
The patches in this series fix the problems I was able to solve, but we
still have the following issues:
1. Our 'private extension' is open to the public
Eg. The following input issued by a client is valid:
{ 'execute': 'query-pci' }
I don't think it's a good idea to have clients relying on this kind of
JSON extension.
To fix this we could add a 'extension' flag to JSONLexer and set it to
nonzero in internal functions (eg. qobject_from_jsonf()), of course that
the lexer code should handle this too.
The JSON specification explicitly says:
"A JSON parser transforms a JSON text into another representation. A
JSON parser MUST accept all texts that conform to the JSON grammar. A
JSON parser MAY accept non-JSON forms or extensions."
IOW, we're under no obligation to reject extensions and I can't think of
a reason why we should.
2. QMP doesn't check the return of json_message_parser_feed()
Which means we don't handle JSON syntax errors. While the fix might seem
trivial (ie. just return an error!), I'm not sure what's the best way
to handle this, because the streamer seems to return multiple errors for
the same input string.
For example, this input:
{ "execute": yy_uu }
Seems to return an error for each bad character (yy_uu), shouldn't it
return only once and stop processing the whole string?
It probably should kill the connection.
3. The lexer enter in ERROR state when processing is done
Not sure whether this is an issue, but I found it while reviewing the code
and maybe this is related with item 2 above.
When json_lexer_feed_char() is finished scanning a string, (ie. ch='\0')
the JSON_SKIP clause will set lexer->state to ERROR as there's no entry
for '\0' in the IN_START array.
Shouldn't we have a LEXER_DONE or something like it instead?
No, you must have malformed input if an error occurs.
[IN_WHITESPACE] -> TERMINAL(JSON_SKIP)
JSON_SKIP is a terminal so once you're in that state, you go back to
IN_START.
4. Lexer expects a 'terminal' char to process a token
Which means clients must send a sort of end of line char, so that we
process their input.
Maybe I'm missing something here, but I thought that the whole point of
writing our own parser was to avoid this.
If the lexer gets:
"abc"
It has no way of knowing if that's a token or if we're going to get:
"abcd"
As a token. You can fix this in two ways. You can either flush() the
lexer to significant end of input or you can wait until there's some
other valid symbol to cause the previous symbol to be emitted.
IOW, a client either needs to: 1) send the request and follow it with a
newline or some form of whitespace or 2) close the connection to flush
the request
Regards,
Anthony Liguori
- [Qemu-devel] Re: [PATCH 2/6] json-lexer: Handle missing escapes, (continued)
- [Qemu-devel] Re: [PATCH 2/6] json-lexer: Handle missing escapes, Anthony Liguori, 2010/05/20
- [Qemu-devel] Re: [PATCH 2/6] json-lexer: Handle missing escapes, Luiz Capitulino, 2010/05/20
- [Qemu-devel] Re: [PATCH 2/6] json-lexer: Handle missing escapes, Anthony Liguori, 2010/05/20
- [Qemu-devel] Re: [PATCH 2/6] json-lexer: Handle missing escapes, Luiz Capitulino, 2010/05/20
- Re: [Qemu-devel] Re: [PATCH 2/6] json-lexer: Handle missing escapes, Anthony Liguori, 2010/05/24
- Re: [Qemu-devel] Re: [PATCH 2/6] json-lexer: Handle missing escapes, Luiz Capitulino, 2010/05/24
[Qemu-devel] [PATCH 3/6] qjson: Handle "\f", Luiz Capitulino, 2010/05/19
[Qemu-devel] [PATCH 4/6] check-qjson: Add more escape tests, Luiz Capitulino, 2010/05/19
[Qemu-devel] [PATCH 6/6] json-streamer: Don't use qdict_put_obj(), Luiz Capitulino, 2010/05/19
[Qemu-devel] [PATCH 5/6] json-lexer: Drop 'buf', Luiz Capitulino, 2010/05/19
Re: [Qemu-devel] [PATCH 0/6]: QMP: Fix issues in parser/lexer,
Anthony Liguori <=
- Re: [Qemu-devel] [PATCH 0/6]: QMP: Fix issues in parser/lexer, Luiz Capitulino, 2010/05/20
- [Qemu-devel] Re: [PATCH 0/6]: QMP: Fix issues in parser/lexer, Paolo Bonzini, 2010/05/20
- [Qemu-devel] Re: [PATCH 0/6]: QMP: Fix issues in parser/lexer, Luiz Capitulino, 2010/05/20
- [Qemu-devel] Re: [PATCH 0/6]: QMP: Fix issues in parser/lexer, Anthony Liguori, 2010/05/20
- [Qemu-devel] Re: [PATCH 0/6]: QMP: Fix issues in parser/lexer, Luiz Capitulino, 2010/05/20
- [Qemu-devel] [PATCH] do not require lookahead in json-lexer.c if not necessary, Paolo Bonzini, 2010/05/21
- [Qemu-devel] [PATCH] do not require lookahead for escapes too, Paolo Bonzini, 2010/05/21
- [Qemu-devel] Re: [PATCH] do not require lookahead for escapes too, Paolo Bonzini, 2010/05/23
Re: [Qemu-devel] [PATCH 0/6]: QMP: Fix issues in parser/lexer, Avi Kivity, 2010/05/20