[PATCH v4 1/2] lib/string_helpers.c: fix infinite loop in string_get_size()

From: Vitaly Kuznetsov
Date: Tue Sep 15 2015 - 09:55:48 EST


Some string_get_size() calls (e.g.:
string_get_size(1, 512, STRING_UNITS_10, ..., ...)
string_get_size(15, 64, STRING_UNITS_10, ..., ...)
) result in an infinite loop. The problem is that if size is equal to
divisor[units]/blk_size and is smaller than divisor[units] we'll end
up with size == 0 when we start doing sf_cap calculations:

For string_get_size(1, 512, STRING_UNITS_10, ..., ...) case:
...
remainder = do_div(size, divisor[units]); -> size is 0, remainder is 1
remainder *= blk_size; -> remainder is 512
...
size *= blk_size; -> size is still 0
size += remainder / divisor[units]; -> size is still 0

The caller causing the issue is sd_read_capacity(), the problem was noticed
on Hyper-V, such weird size was reported by host when scanning collides
with device removal. This is probably a separate issue worth fixing, this
patch is intended to prevent the library routine from infinite looping.

Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
Acked-by: James Bottomley <JBottomley@xxxxxxxx>
---
lib/string_helpers.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/lib/string_helpers.c b/lib/string_helpers.c
index c98ae81..bbf1bef 100644
--- a/lib/string_helpers.c
+++ b/lib/string_helpers.c
@@ -59,7 +59,11 @@ void string_get_size(u64 size, u64 blk_size, const enum string_size_units units,
}

exp = divisor[units] / (u32)blk_size;
- if (size >= exp) {
+ /*
+ * size must be strictly greater than exp here to ensure that remainder
+ * is greater than divisor[units] coming out of the if below.
+ */
+ if (size > exp) {
remainder = do_div(size, divisor[units]);
remainder *= blk_size;
i++;
--
2.4.3

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