[PATCH] improve speed and correctness of GUID string parsing

Andrew Bartlett abartlet at samba.org
Fri Dec 9 09:17:28 UTC 2016

On Fri, 2016-12-09 at 09:30 +1300, Douglas Bagnall wrote:
> In the code that parses guid strings in librpc/ndr/uuid.c (and other
> places), we accept a large family of guid-like non-guids as guids.
> This patch stops that and makes the parsing of true guids much
> faster.
> The old code used sscanf() where we appear to mistakenly expect
> formats
> like "%04x" to match only the strings that printf() would produce
> when
> given it (in this case four hex characters padded with leading zeros,
> like "abcd" or "0002"). But sscanf doesn't work like that: the "0" is
> interpreted as part of the number, not as a flag, and it matches any
> sequence of up to four characters that could possibly be a hex
> number.
> That includes strings like " 033", "0x2A", "-4 ", which (though I
> admit to not looking up RFCs) are not normally expected to occur in
> GUIDs.
> We never noticed this because our test for invalid guids only tried a
> single un-guid-like string ("bla").
> I came to this chasing performance. In my ldap.ad_dc_performance
> tests
> parsing GUIDs was taking 7.8% of the overall time. These patches
> reduce that to 2.8%.
> I see similar GUID routines in nsswitch/libwbclient/wbc_guid.c and
> source4/dsdb/common/util.c, and many other places where we use "%0"
> in
> sscanf expecting it to mean something.

This is really, really good work!

Reviewed-by: Andrew Bartlett <abartlet at samba.org>

Please just fix the line-wrap in the functions.  I think you already
fixed it up in a private tree at the office, but we don't put the
static and type on a line before the function name in Samba.


Andrew Bartlett
Andrew Bartlett                       http://samba.org/~abartlet/
Authentication Developer, Samba Team  http://samba.org
Samba Developer, Catalyst IT          http://catalyst.net.nz/services/samba

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