[PATCH] bcache: prevent potential division by zero error

[PATCH] bcache: prevent potential division by zero error

[Rand Deeb]

In SHOW(), the variable 'n' is of type 'size_t.' While there is a
conditional check to verify that 'n' is not equal to zero before
executing the 'do_div' macro, concerns arise regarding potential
division by zero error in 64-bit environments.

The concern arises when 'n' is 64 bits in size, greater than zero, and
the lower 32 bits of it are zeros. In such cases, the conditional check
passes because 'n' is non-zero, but the 'do_div' macro casts 'n' to
'uint32_t,' effectively truncating it to its lower 32 bits.
Consequently, the 'n' value becomes zero.

To fix this potential division by zero error and ensure precise
division handling, this commit replaces the 'do_div' macro with
div64_u64(). div64_u64() is designed to work with 64-bit operands,
guaranteeing that division is performed correctly.

This change enhances the robustness of the code, ensuring that division
operations yield accurate results in all scenarios, eliminating the
possibility of division by zero, and improving compatibility across
different 64-bit environments.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Signed-off-by: Rand Deeb <rand.sec96@gmail.com>
---
 drivers/md/bcache/sysfs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/md/bcache/sysfs.c b/drivers/md/bcache/sysfs.c
index 554e3afc9b68..ca3e2f000cd4 100644
--- a/drivers/md/bcache/sysfs.c
+++ b/drivers/md/bcache/sysfs.c
@@ -1078,7 +1078,7 @@ SHOW(__bch_cache)
 			sum += INITIAL_PRIO - cached[i];
 
 		if (n)
-			do_div(sum, n);
+			sum = div64_u64(sum, n);
 
 		for (i = 0; i < ARRAY_SIZE(q); i++)
 			q[i] = INITIAL_PRIO - cached[n * (i + 1) /
-- 
2.34.1

Re: [PATCH] bcache: prevent potential division by zero error

[Coly Li]

> 2023年9月6日 09:22，Rand Deeb <rand.sec96@gmail.com> 写道：
> 
> In SHOW(), the variable 'n' is of type 'size_t.' While there is a
> conditional check to verify that 'n' is not equal to zero before
> executing the 'do_div' macro, concerns arise regarding potential
> division by zero error in 64-bit environments.
> 
> The concern arises when 'n' is 64 bits in size, greater than zero, and
> the lower 32 bits of it are zeros. In such cases, the conditional check
> passes because 'n' is non-zero, but the 'do_div' macro casts 'n' to
> 'uint32_t,' effectively truncating it to its lower 32 bits.
> Consequently, the 'n' value becomes zero.
> 
> To fix this potential division by zero error and ensure precise
> division handling, this commit replaces the 'do_div' macro with
> div64_u64(). div64_u64() is designed to work with 64-bit operands,
> guaranteeing that division is performed correctly.
> 
> This change enhances the robustness of the code, ensuring that division
> operations yield accurate results in all scenarios, eliminating the
> possibility of division by zero, and improving compatibility across
> different 64-bit environments.
> 
> Found by Linux Verification Center (linuxtesting.org) with SVACE.
> 
> Signed-off-by: Rand Deeb <rand.sec96@gmail.com>

Thanks, added into my for-next queue.

Coly Li

> ---
> drivers/md/bcache/sysfs.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/md/bcache/sysfs.c b/drivers/md/bcache/sysfs.c
> index 554e3afc9b68..ca3e2f000cd4 100644
> --- a/drivers/md/bcache/sysfs.c
> +++ b/drivers/md/bcache/sysfs.c
> @@ -1078,7 +1078,7 @@ SHOW(__bch_cache)
> sum += INITIAL_PRIO - cached[i];
> 
> if (n)
> - do_div(sum, n);
> + sum = div64_u64(sum, n);
> 
> for (i = 0; i < ARRAY_SIZE(q); i++)
> q[i] = INITIAL_PRIO - cached[n * (i + 1) /
> -- 
> 2.34.1
>