* [PATCH v2] lib/string_helpers.c: fix infinite loop in string_get_size()
@ 2015-09-14 12:57 Vitaly Kuznetsov
2015-09-14 15:15 ` James Bottomley
0 siblings, 1 reply; 2+ messages in thread
From: Vitaly Kuznetsov @ 2015-09-14 12:57 UTC (permalink / raw)
To: Andrew Morton
Cc: Andy Shevchenko, Rasmus Villemoes, James Bottomley, linux-kernel,
K. Y. Srinivasan
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 routing from infinite looping.
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
lib/string_helpers.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/lib/string_helpers.c b/lib/string_helpers.c
index c98ae81..d0e607c 100644
--- a/lib/string_helpers.c
+++ b/lib/string_helpers.c
@@ -59,7 +59,7 @@ void string_get_size(u64 size, u64 blk_size, const enum string_size_units units,
}
exp = divisor[units] / (u32)blk_size;
- if (size >= exp) {
+ if (size > exp) {
remainder = do_div(size, divisor[units]);
remainder *= blk_size;
i++;
--
2.4.3
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH v2] lib/string_helpers.c: fix infinite loop in string_get_size()
2015-09-14 12:57 [PATCH v2] lib/string_helpers.c: fix infinite loop in string_get_size() Vitaly Kuznetsov
@ 2015-09-14 15:15 ` James Bottomley
0 siblings, 0 replies; 2+ messages in thread
From: James Bottomley @ 2015-09-14 15:15 UTC (permalink / raw)
To: vkuznets@redhat.com
Cc: linux@rasmusvillemoes.dk, andriy.shevchenko@linux.intel.com,
linux-kernel@vger.kernel.org, akpm@linux-foundation.org,
kys@microsoft.com
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On Mon, 2015-09-14 at 14:57 +0200, Vitaly Kuznetsov wrote:
> 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 routing from infinite looping.
>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
> lib/string_helpers.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/lib/string_helpers.c b/lib/string_helpers.c
> index c98ae81..d0e607c 100644
> --- a/lib/string_helpers.c
> +++ b/lib/string_helpers.c
> @@ -59,7 +59,7 @@ void string_get_size(u64 size, u64 blk_size, const enum string_size_units units,
> }
>
> exp = divisor[units] / (u32)blk_size;
Add a comment here explaining to those who come after why strict greater
than is necessary. Something like:
/* size must be strictly greater than exp here to ensure
* that remainder is greater than divisor[units] coming out of the if */
> - if (size >= exp) {
> + if (size > exp) {
> remainder = do_div(size, divisor[units]);
> remainder *= blk_size;
> i++;
So when pushed, you finally found the algorithmic problem. Well done.
Acked-by: James Bottomley <JBottomley@Odin.com>
James
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2015-09-14 12:57 [PATCH v2] lib/string_helpers.c: fix infinite loop in string_get_size() Vitaly Kuznetsov
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