linux-ext4 - [Bug 114701] ubsan: "shift exponent -1 is negative" in fs/ext4/mballoc.c:2612:15

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Message-ID: <bug-114701-13602-vrpyELHlhR@https.bugzilla.kernel.org/>
Date:	Wed, 30 Mar 2016 09:12:29 +0000
From:	bugzilla-daemon@...zilla.kernel.org
To:	linux-ext4@...r.kernel.org
Subject: [Bug 114701] ubsan: "shift exponent -1 is negative" in
 fs/ext4/mballoc.c:2612:15

https://bugzilla.kernel.org/show_bug.cgi?id=114701

Navin Parakkal <navinp1912@...il.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |navinp1912@...il.com

--- Comment #1 from Navin Parakkal <navinp1912@...il.com> ---
diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 50e05df..8ccfcf7 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -1278,6 +1278,8 @@ static int mb_find_order_for_block(struct ext4_buddy
*e4b, int block)
                        /* this block is part of buddy of order 'order' */
                        return order;
                }
+               if (order > e4b->bd_blkbits)
+                       break;
                bb += 1 << (e4b->bd_blkbits - order);
                order++;
        }
@@ -2616,6 +2618,8 @@ int ext4_mb_init(struct super_block *sb)
        do {
                sbi->s_mb_offsets[i] = offset;
                sbi->s_mb_maxs[i] = max;
+               if (i > sb->s_blocksize_bits)
+                       break;
                offset += 1 << (sb->s_blocksize_bits - i);
                max = max >> 1;
                i++;


The only case if when i>b you compute off and max but those are not assigned .
So those values can be ignored. It keeps the old behaviour intact and fixes the
undefined behaviour.


offset += 1 << (sb->s_blocksize_bits - i);


/* Remove the if to see trap error at runtime */
/* gcc -fsanitize=undefined x.c */
int main()
{
    int i=1;
    int b=12;
    unsigned ub=12;
    unsigned off=0,uoff=0;
    do{
        if(i> b) break;
        off+=1<<(b-i);
        uoff+=1<<(ub-i);
        i++;
    } while(i<=b+1);
}

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