Subject: Re: Bug in 'git am' when applying a broken patch

From: Junio C Hamano <[email-masked]>
Date: Mon, 01 Jun 2015 11:31:10 -0700

Greg KH <[email-masked]> writes:

> But, there's nothing in the patch at all except the commit message:
>
> $ git show HEAD
> ...
> Any ideas what is going on here?  Shouldn't 'git am' have failed?

Yes.  The patch reads like this:

    ---
     drivers/staging/rtl8192u/ieee80211/ieee80211_softmac.c | 5 +++--
     1 file changed, 3 insertions(+), 2 deletions(-)

    diff --git a/drivers/staging/rtl8192u/ieee80211/ieee80211_softmac.c b/...
    index d2e8b12..0477ba1 100644
    --- a/drivers/staging/rtl8192u/ieee80211/ieee80211_softmac.c
    +++ b/drivers/staging/rtl8192u/ieee80211/ieee80211_softmac.c
    @@ -660,2 +660,2 @@ inline struct sk_buff *ieee80211_authentic...
            auth = (struct ieee80211_authentication *)
                    skb_put(skb, sizeof(struct ieee80211_authentication));

    -	auth->header.frame_ctl = IEEE80211_STYPE_AUTH;
    -	if (challengelen) auth->header.frame_ctl |= IEEE80211_FCTL_WEP;
    +	auth->header.frame_ctl = cpu_to_le16(IEEE80211_STYPE_AUTH);
    +	if (challengelen)
    +		auth->header.frame_ctl |= cpu_to_le16(IEEE80211_FCTL_WEP);

            auth->header.duration_id = 0x013a; //FIXME

    --
    2.1.4

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It claims that it has only 2 lines in the hunk, so "git apply"
parses the hunk that begins at line 660 as such:

    @@ -660,2 +660,2 @@ inline struct sk_buff *ieee80211_authentic...
            auth = (struct ieee80211_authentication *)
                    skb_put(skb, sizeof(struct ieee80211_authentication));

And then seeing that the next line (which is a blank line, not even
a lone SP on it) does not begin with "@@ -", it says "OK, the
remainder is a cruft after the patch" and discards the rest (which
it must be capable of, to ignore "-- ", "2.1.4", "devel mailing
list", etc.)

There is some safety against not finding a correct patch header
(i.e. "diff --git" line) by detecting a lone "@@ -" while parsing
the patch stream, but there is no logic implemented to detect this
kind of breakage in the code.