Re: [PATCH 05/10] packed-backend: check whether the refname contains NULL binaries
From: Patrick Steinhardt <hidden>
Date: 2025-01-16 13:57:43
On Sun, Jan 05, 2025 at 09:49:51PM +0800, shejialuo wrote:
We have already implemented the header consistency check for the raw
"packed-refs" file. Before we implement the consistency check for each
ref entry, let's analysis [1] which reports that "git fsck" cannot
detect some binary zeros.
"packed-backend.c::next_record" will use "check_refname_format" to check
the consistency of the refname. If it is not OK, the program will die.
So, we already have the code path and we must miss out something.
We use the following code to get the refname:
strbuf_add(&iter->refname_buf, p, eol - p);
iter->base.refname = iter->refname_buf.buf
In the above code, `p` is the start pointer of the refname and `eol` is
the next newline pointer. We calculate the length of the refname by
subtracting the two pointers. Then we add the memory range between `p`
and `eol` to get the refname.
However, if there are some NULL binaries in the memory range between `p`You probably mean NUL characters, not NULL binaries?
quoted hunk ↗ jump to hunk
diff --git a/refs/packed-backend.c b/refs/packed-backend.c index 3b11abe5f8..f6142a4402 100644 --- a/refs/packed-backend.c +++ b/refs/packed-backend.c@@ -493,6 +493,23 @@ static void verify_buffer_safe(struct snapshot *snapshot) last_line, eof - last_line); } +/* + * When parsing the "packed-refs" file, we will parse it line by line. + * Because we know the start pointer of the refname and the next + * newline pointer, we could calculate the length of the refname by + * subtracting the two pointers. However, there is a corner case where + * the refname contains corrupted embedded NULL binaries. And + * `check_refname_format()` will not catch this when the truncated + * refname is still a valid refname. To prevent this, we need to check + * whether the refname contains the NULL binaries. + */ +static int refname_contains_null(struct strbuf refname) +{ + if (refname.len != strlen(refname.buf)) + return 1; + return 0; +} + #define SMALL_FILE_SIZE (32*1024) /*@@ -894,6 +911,9 @@ static int next_record(struct packed_ref_iterator *iter) strbuf_add(&iter->refname_buf, p, eol - p); iter->base.refname = iter->refname_buf.buf; + if (refname_contains_null(iter->refname_buf))
We can replace this with `memchr(iter->refname_buf.buf, '\0', iter->refname_buf.len)`, which should be more efficient than using strlen(3p).
+ die("packed refname contains embedded NULL: %s", iter->base.refname);
+I was a bit surprised to find that we modify the way that we read refs from the packed-refs file instead of adapting the fsck code. But I think this check is sensible. Patrick