2019-02-23 08:51:45

by Hongbo Yao

Subject: [PATCH] fs: Avoid signed overflow UBSAN warning

I ran into this:
UBSAN: Undefined behaviour in fs/read_write.c:251:11
signed integer overflow:
9223372036854775805 + 9223372036854775805 cannot be represented in
type 'long long int'

Use unsigned types to do math, it doesn't throw a UBSAN warning
when it does overflow.This patch doesn't change generated code.

Signed-off-by: Hongbo Yao <[email protected]>
---
fs/read_write.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/read_write.c b/fs/read_write.c
index 8f08d4a2aa9a..0509e81afc8e 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -248,7 +248,7 @@ loff_t default_llseek(struct file *file, loff_t offset, int whence)
retval = file->f_pos;
goto out;
}
- offset += file->f_pos;
+ offset = (u64)file->f_pos + (u64)offset;
break;
case SEEK_DATA:
/*
--
2.20.1

2019-03-05 04:27:10

by Matthew Wilcox

Subject: Re: [PATCH] fs: Avoid signed overflow UBSAN warning

On Sat, Feb 23, 2019 at 05:02:10PM +0800, Hongbo Yao wrote:
> I ran into this:
> UBSAN: Undefined behaviour in fs/read_write.c:251:11
> signed integer overflow:
> 9223372036854775805 + 9223372036854775805 cannot be represented in
> type 'long long int'
>
> Use unsigned types to do math, it doesn't throw a UBSAN warning
> when it does overflow.This patch doesn't change generated code.

You don't need to be this verbose to fix it.

> - offset += file->f_pos;
> + offset = (u64)file->f_pos + (u64)offset;

offset += (u64)file->f_pos;