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Re: [PATCH v2 2/2] block/rbd: Add an escape-aware strchr helper

From: Max Reitz
Subject: Re: [PATCH v2 2/2] block/rbd: Add an escape-aware strchr helper
Date: Tue, 6 Apr 2021 16:24:36 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0

On 01.04.21 23:01, Connor Kuehl wrote:
Sometimes the parser needs to further split a token it has collected
from the token input stream. Right now, it does a cursory check to see
if the relevant characters appear in the token to determine if it should
break it down further.

However, qemu_rbd_next_tok() will escape characters as it removes tokens
from the token stream and plain strchr() won't. This can make the
initial strchr() check slightly misleading since it implies
qemu_rbd_next_tok() will find the token and split on it, except the
reality is that qemu_rbd_next_tok() will pass over it if it is escaped.

Use a custom strchr to avoid mixing escaped and unescaped string

Reported-by: Han Han <hhan@redhat.com>
Fixes: https://bugzilla.redhat.com/1873913
Signed-off-by: Connor Kuehl <ckuehl@redhat.com>
  block/rbd.c                | 20 ++++++++++++++++++--
  tests/qemu-iotests/231     |  4 ++++
  tests/qemu-iotests/231.out |  3 +++
  3 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/block/rbd.c b/block/rbd.c
index 9071a00e3f..c0e4d4a952 100644
--- a/block/rbd.c
+++ b/block/rbd.c
@@ -134,6 +134,22 @@ static char *qemu_rbd_next_tok(char *src, char delim, char 
      return src;
+static char *qemu_rbd_strchr(char *src, char delim)
+    char *p;
+    for (p = src; *p; ++p) {
+        if (*p == delim) {
+            return p;
+        }
+        if (*p == '\\') {
+            ++p;
+        }
+    }
+    return NULL;

So I thought you could make qemu_rbd_do_next_tok() to do this. (I didn’t say you should, but bear with me.) That would be possible by giving it a new parameter (e.g. @find), and if that is set, return @end if *end == delim after the loop, and NULL otherwise.

Now, if you add wrapper functions to make it nice, there’s not much more difference in lines added compared to just adding a new function, but it does mean your function should basically be the same as qemu_rbd_next_tok(), except that no splitting happens, that there is no *p, and that @end is returned instead of @src.

So there is one difference, and that is that qemu_rbd_next_tok() has this condition to skip escaped characters:

    if (*end == '\\' && end[1] != '\0') {

where qemu_rbd_strchr() has only:

    if (*p == '\\') {

And I think qemu_rbd_next_tok() is right; if the string in question has a trailing backslash, qemu_rbd_strchr() will ignore the final NUL and continue searching past the end of the string.


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