netdev - [iproute2 1/2] ss: fix crash when skipping disabled header field

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Message-Id: <20180106183150.10530-1-a@unstable.cc>
Date:   Sun,  7 Jan 2018 02:31:49 +0800
From:   Antonio Quartulli <a@...table.cc>
To:     netdev@...r.kernel.org
Cc:     Antonio Quartulli <a@...table.cc>,
        Stefano Brivio <sbrivio@...hat.com>,
        Stephen Hemminger <stephen@...workplumber.org>
Subject: [iproute2 1/2] ss: fix crash when skipping disabled header field

When the first header field is disabled (i.e. when passing the -t
option), field_flush() is invoked with the `buffer` global variable
still zero'd.
However, in field_flush() we try to access buffer.cur->len
during variables initialization, thus leading to a SIGSEGV.

It's interesting to note that this bug appears only when the code
is compiled with -O0, because the compiler is smart
enough to immediately jump to the return statement if optimizations
are enabled and skip the faulty instruction.

Cc: Stefano Brivio <sbrivio@...hat.com>
Cc: Stephen Hemminger <stephen@...workplumber.org>
Signed-off-by: Antonio Quartulli <a@...table.cc>
---
 misc/ss.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/misc/ss.c b/misc/ss.c
index 1abf43d0..b35859dc 100644
--- a/misc/ss.c
+++ b/misc/ss.c
@@ -1018,12 +1018,15 @@ static void print_right_spacing(struct column *f, int printed)
 /* Done with field: update buffer pointer, start new token after current one */
 static void field_flush(struct column *f)
 {
-	struct buf_chunk *chunk = buffer.tail;
-	unsigned int pad = buffer.cur->len % 2;
+	struct buf_chunk *chunk;
+	unsigned int pad;
 
 	if (f->disabled)
 		return;
 
+	chunk = buffer.tail;
+	pad = buffer.cur->len % 2;
+
 	if (buffer.cur->len > f->max_len)
 		f->max_len = buffer.cur->len;
 
-- 
2.15.1