Json log for v2.4

[thomasmey]

No description provided.

[notroj]

Interesting idea, thanks for the submission! Some comments:

• some code style issues and C++-style comments, our style guide is at https://httpd.apache.org/dev/styleguide.html
• it looks like you need to now make the log_format_item struct visible outside mod_log_config so it's usable in the new module, but I don't see where that is happening? it shouldn't be exported as void * anyway

Approving so we get a CI run.

[ylavic]

items shouldn't be changed by the writers so const void *items for the 3 above?

[ylavic]

const char *tag, ditto for the existing char *arg below though it does not originate from this commit.

[ylavic]

Possibly this most common case should be tested first?

[ylavic]

There could be a json_code_point_needs_encoding() helper since this same check is in json_encode() below too.

[ylavic]

Maybe 0x7f (another control character) is missing?

[thomasmey]

Strangley 0x7f is not part of the codepoints that needs escaping in JSON strings, see https://www.rfc-editor.org/rfc/rfc8259#page-9
Searching for JSON and 0x7f seems to at least reveal on bug report dominictarr/JSON.sh#46

[ylavic]

I think 0x7f is covered in https://www.rfc-editor.org/rfc/rfc8259#section-7

   All Unicode characters may be placed within the
   quotation marks, except for the characters that MUST be escaped:
   quotation mark, reverse solidus, and the control characters (U+0000
   through U+001F).

[ylavic]

Hm, scratch that, it stops at U+001F (not U+007F). TIL..

[ylavic]

VLA (Variable Length Array) should be avoided in httpd, you could use an apr_array_header_t of struct iovec and modify the add_iovec_str() to push there instead.

[ylavic]

Using apr_file_writev_full() directly with the iovec array would avoid allocating json_str here.

[yakatz]

Since JSON is already pretty verbose, it might be nice to have an option to make the keys be the single chars from the format definition and not be converted back to the attribute name.

[yakatz]

What about having both kinds of logs at once - for example, maybe beginning the LogFormat line with json: or an additional flag to the LogFormat directive that could specify an (arbitrary) format. This would be useful to allow migration and further output options.

[thomasmey]

hi, @yakatz, @notroj and @ylavic do you prefer rebase, i.e. incorporate the fixes in the existing commits or should I add more commits to better follow the change history?

And one general question regarding the UTF-8 handling in JSON: After digging a bit more into the code I'm not sure if this is necessary at all in this form...

What charset is are the strings given to mod_log_config are in? All "extractor" function do call ap_escape_logitem() in the end which do seem to use an 8-bit index to deceide if to escape or not, so the assumption is to have an 8-bit fixed charater set at this stage?
is the charset documented somewhere?
so maybe I need to convert to UTF-8 instead of doing escaping?
and another question: why does ap_escape_logitem() return a copy of the input string even if no escpaing is done, bascially duplicates all strings in mod_log_config?

[ylavic]

I (personnaly) think rebasing is fine in PRs/feature branches.

Regarding the encoding, you are right that ap_escape_logitem() should already \-escape controls, double quote and non-ascii characters, so there is nothing left to json_needs_encoding() currently (besides \ itself because this \-escaping is not json compatible..).
Whether that's the right encoding to use for json I'm not sure either, there is necessarily an encoding to apply (in addition to json) if we want a preserving/reversible encoding because HTTP is not necessarily UTF-8 (could be ISO-8859-1, sometimes CP-1252..). But ap_escape_logitem() will escape valid UTF-8 (in headers for instance) which may be unfortunate.
Maybe that's the right thing to do because SIEMs / log managers may already know how to unescape ap_escape_logitem(), though probably not in json strings, but possibly we want to preserve UTF-8 if it's there already.
So possibly we want a all-in-one ap_escape_logjson() to replace ap_escape_logitem() in json logging mode, which would let valid UTF-8 through and for non-UTF-8 characters we could either:

1. use the unicode replacement character U+FFFD (non preserving obviously)
2. assume latin1/ISO-8859-1 and do a trivial code point to UTF-8 conversion (that's non preserving though if the source is mixed with native UTF-8, and latin1 extended control characters get no special encoding)
3. assume latin1/ISO-8859-1 and print them as \u00xx (visually different though semantically the same as 2.)
4. \x-escape them, and \ itself too then (preserving, but a lot of \ since json requires it to be escaped already)
5. ...

I think I like 3. better (assuming ISO-8859-1 is not that insane IMO), but it's non strictly preserving. Or 4. What do you/others think?

[ylavic]

another question: why does ap_escape_logitem() return a copy of the input string even if no escpaing is done, bascially duplicates all strings in mod_log_config?

It returns a char *, so it can't return the const char *str source directly.
There could be a const char *ap_escape_logitem_c(...) or something that returns the original string if nothing needs escaping though, but we can't change the API for the existing function.
Maybe it's something to think about for an eventual new ap_escape_logjson() function..

[ylavic]

I was thinking something like this for ap_escape_logjson():

/* See http://bjoern.hoehrmann.de/utf-8/decoder/dfa/ for details. */
#define DECODE_UTF8_ACCEPT 0
#define DECODE_UTF8_REJECT 1
static const unsigned char decode_utf8d[] = {
    0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, /* 00..1f */
    0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, /* 20..3f */
    0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, /* 40..5f */
    0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, /* 60..7f */
    1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9, /* 80..9f */
    7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7, /* a0..bf */
    8,8,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2, /* c0..df */
    0xa,0x3,0x3,0x3,0x3,0x3,0x3,0x3,0x3,0x3,0x3,0x3,0x3,0x4,0x3,0x3, /* e0..ef */
    0xb,0x6,0x6,0x6,0x5,0x8,0x8,0x8,0x8,0x8,0x8,0x8,0x8,0x8,0x8,0x8, /* f0..ff */
    0x0,0x1,0x2,0x3,0x5,0x8,0x7,0x1,0x1,0x1,0x4,0x6,0x1,0x1,0x1,0x1, /* s0..s0 */
    1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,0,1,1,1,1,1,0,1,0,1,1,1,1,1,1, /* s1..s2 */
    1,2,1,1,1,1,1,2,1,2,1,1,1,1,1,1,1,1,1,1,1,1,1,2,1,1,1,1,1,1,1,1, /* s3..s4 */
    1,2,1,1,1,1,1,1,1,2,1,1,1,1,1,1,1,1,1,1,1,1,1,3,1,3,1,1,1,1,1,1, /* s5..s6 */
    1,3,1,1,1,1,1,3,1,3,1,1,1,1,1,1,1,3,1,1,1,1,1,1,1,1,1,1,1,1,1,1, /* s7..s8 */
};
static APR_INLINE apr_uint32_t decode_utf8_byte(apr_uint32_t state,
                                                apr_uint32_t *codep,
                                                unsigned char c)
{
    apr_uint32_t byte = c;
    apr_uint32_t type = decode_utf8d[byte];

    if (codep) {
        *codep = (state != DECODE_UTF8_ACCEPT) ?
            (byte & 0x3fu) | (*codep << 6) :
            (0xff >> type) & (byte);
    }

    return decode_utf8d[256 + state * 16 + type];
}

static APR_INLINE unsigned char *escape_json_byte(unsigned char d[6],
                                                  unsigned char c)
{
    apr_uint32_t byte = c;
    d[0] = '\\';
    d[1] = 'u';
    d[2] = '0';
    d[3] = '0';
    d[4] = c2x_table[byte >> 4];
    d[5] = c2x_table[byte & 15];
    return d + 6;
}

AP_DECLARE(const char *) ap_escape_logjson(apr_pool_t *p, const char *str,
                                           apr_size_t *len, int quoted)
{
    const unsigned char *s = (const unsigned char *)str;
    apr_size_t seq, pos, length, escapes = 0;
    apr_uint32_t state, prev;
    unsigned char *d;
    char *ret;

    /* Compute how many characters need to be escaped */
    state = prev = DECODE_UTF8_ACCEPT;
    length = (len && *len != APR_SIZE_MAX) ? *len : (str) ? strlen(str) : 0;
    for (seq = pos = 0; pos < length; ++pos, prev = state) {
        unsigned char c = s[pos];
        state = decode_utf8_byte(state, NULL, c);
        switch (state) {
        case DECODE_UTF8_ACCEPT:
            switch (c) {
            case '\b': case '\t': case '\n':
            case '\f': case '\r': case '\\':
            case '"':
                /* \-escape these */
                escapes++;
                break;
            default:
                /* \u-escape controls (U+0000 through U+001F) */
                if (c < 0x20) {
                    escapes += 5;
                }
            }
            seq = pos + 1;
            break;

        case DECODE_UTF8_REJECT:
            if (prev == DECODE_UTF8_ACCEPT) {
                /* \u-escape invalid [pos] (only) */
                escapes += 5;
                seq++;
            }
            else {
                /* \u-escape each byte of invalid [seq,pos[ sequence and
                 * let above decide for [pos]
                 */
                if (seq < pos) {
                    escapes += (pos - seq) * 5;
                    seq = pos;
                }
                pos--;
            }
            state = DECODE_UTF8_ACCEPT;
            break;

        default:
            /* valid continuation byte */
            break;
        }
    }
    if (seq < length) {
        /* \u-escape each byte of unterminated [seq,length[ sequence */
        escapes += (length - seq) * 5;
    }

    /* Fast path: nothing to escape */
    if (escapes == 0) {
        if (quoted) {
            if (str) {
                str = apr_pstrcat(p, "\"", str, "\"", NULL);
                length += 2;
            }
            else {
                str = "null";
                length = 4;
            }
        }
        if (len) {
            *len = length;
        }
        return str;
    }

    /* Do the actual escaping in a suitable buffer */
    state = prev = DECODE_UTF8_ACCEPT;
    ret = apr_palloc(p, length + escapes + (quoted ? 2 : 0) + 1);
    d = (unsigned char *)ret;
    if (quoted) {
        *d++ = '"';
    }
    for (seq = pos = 0; pos < length; ++pos, prev = state) {
        unsigned char c = s[pos];
        state = decode_utf8_byte(state, NULL, c);
        switch (state) {
        case DECODE_UTF8_ACCEPT:
            switch (c) {
            case '\b':
                *d++ = '\\';
                *d++ = 'b';
                break;
            case '\t':
                *d++ = '\\';
                *d++ = 't';
                break;
            case '\n':
                *d++ = '\\';
                *d++ = 'n';
                break;
            case '\f':
                *d++ = '\\';
                *d++ = 'f';
                break;
            case '\r':
                *d++ = '\\';
                *d++ = 'r';
                break;
            case '\\':
            case '"':
                *d++ = '\\';
                *d++ = c;
                break;
            default:
                if (c < 0x20) {
                    /* control to \u-escape */
                    d = escape_json_byte(d, c);
                }
                else {
                    /* valid (multi-)byte as is */
                    while (seq < pos) {
                        *d++ = s[seq++];
                    }
                    *d++ = c;
                }
            }
            seq++;
            break;

        case DECODE_UTF8_REJECT:
            if (prev == DECODE_UTF8_ACCEPT) {
                d = escape_json_byte(d, c);
                seq++;
            }
            else {
                while (seq < pos) {
                    d = escape_json_byte(d, s[seq++]);
                }
                pos--;
            }
            state = DECODE_UTF8_ACCEPT;
            break;
        }
    }
    while (seq < length) {
        d = escape_json_byte(d, s[seq++]);
    }
    if (quoted) {
        *d++ = '"';
    }
    if (len) {
        *len = d - (const unsigned char *)ret;
    }
    *d = '\0';

    return ret;
}

Feel free to take/modify/ignore (only compile-tested..).

[thomasmey]

I (personnaly) think rebasing is fine in PRs/feature branches.

Regarding the encoding, you are right that ap_escape_logitem() should already -escape controls, double quote and non-ascii characters, so there is nothing left to json_needs_encoding() currently (besides \ itself because this -escaping is not json compatible..). Whether that's the right encoding to use for json I'm not sure either, there is necessarily an encoding to apply (in addition to json) if we want a preserving/reversible encoding because HTTP is not necessarily UTF-8 (could be ISO-8859-1, sometimes CP-1252..). But ap_escape_logitem() will escape valid UTF-8 (in headers for instance) which may be unfortunate. Maybe that's the right thing to do because SIEMs / log managers may already know how to unescape ap_escape_logitem(), though probably not in json strings, but possibly we want to preserve UTF-8 if it's there already. So possibly we want a all-in-one ap_escape_logjson() to replace ap_escape_logitem() in json logging mode, which would let valid UTF-8 through and for non-UTF-8 characters we could either:

1. use the unicode replacement character `U+FFFD` (non preserving obviously)

2. assume latin1/ISO-8859-1 and do a trivial code point to UTF-8 conversion (that's non preserving though if the source is mixed with native UTF-8, and latin1 extended control characters get no special encoding)

3. assume latin1/ISO-8859-1 and print them as `\u00xx` (visually different though semantically the same as 2.)

4. \x-escape them, and `\` itself too then (preserving, but a lot of `\` since json requires it to be escaped already)

5. ...

I think I like 3. better (assuming ISO-8859-1 is not that insane IMO), but it's non strictly preserving. Or 4. What do you/others think?

Hi,

cool, I had similar ideas to solve this problem, very nice!

I will try to work on all the comments here the next days and thanks for your valuable feedback very appreciated!

But be a bit patient with me as this is purely a hobby project I do in my spare time and depending how stressful my day ways and when the little one falls asleep it may take some time 😄

[thomasmey]

* some code style issues and C++-style comments, our style guide is at https://httpd.apache.org/dev/styleguide.html

running GNU indent on mod_log_config.[ch] (as described in the styleguide) gives interessting results... looks like nobody seems to run indent before each commit; I assume that probably no git hooks, no automation are in place.

I can add one last "GNU ident"-commit in this PR to comply with styleguide rules, but the fallout will be big and will not cover only my changes, if you like.

[thomasmey]

some more ideas inspired by @yakatz. please have a look, it can be activated with:

CustomLog "logs/access_log" combined formatter=json
CustomLog "logs/access_log" combined formatter=json,short

@ylavic I moved json escape function to where is fits better, but the actual escape function you proposed is still a todo, I'll work on this next.

[yakatz]

CustomLog "logs/access_log" combined formatter=json
CustomLog "logs/access_log" combined formatter=json,short

Looks good to me

[thomasmey]

Hi, any update ls on this?
Something else I could do for this PR?

[ylavic]

This needs to go in the caller probably, we don't need it in json for instance (where "" is better than "\"\"" as a value). See my comment in config_log_transaction().

[ylavic]

We don't want empty parts here, so possibly:

    if (*lfd->portions[i]) {
        lfd->portions[i] = ap_escape_logitem(r->pool, lfd->portions[i])
    }
    else {
        lfd->portions[i] = "\"\"";
    }

?

[ylavic]

Maybe even NULL should be handled here, like:

    if (!lfd->portions[i]) {
        lfd->portions[i] = "-";
    }
    else if (!*lfd->portions[i]) {
        lfd->portions[i] = "\"\"";
    }
    else {
        lfd->portions[i] = ap_escape_logitem(r->pool, lfd->portions[i])
    }

This would let the formatter handle it (using the null value in json for instance).

[ylavic]

Maybe do the renaming then? log_writer the function vs log_writer the data is quite confusing..

[ylavic]

It looks quite good to me, but it should be based on trunk rather than 2.4.x (nothing goes to 2.4 without making it to trunk first). Not sure if the target branch can be changed without creating a new PR, sorry for noticing this only now :/

[ylavic]

This code is provided under the MIT license IIUC, so a note in the /LICENSE file should be fine I think. We can probably do that when merging.

[thomasmey]

It looks quite good to me, but it should be based on trunk rather than 2.4.x (nothing goes to 2.4 without making it to trunk first). Not sure if the target branch can be changed without creating a new PR, sorry for noticing this only now :/

updated trunk version: #373

[OkayAndUTry]

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