Re: [PATCH] ceph: Fix potential undefined behavior in crush_ln() with GCC 11.1.0

[Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>]

On Thu, 2025-09-18 at 09:50 +0800, 陈华昭（Lyican） wrote:
> When compiled with GCC 11.1.0 and -march=x86-64-v3 -O1 optimization flags,
> __builtin_clz() may generate BSR instructions without proper zero handling.
> The BSR instruction has undefined behavior when the source operand is zero,
> which could occur when (x & 0x1FFFF) equals 0 in the crush_ln() function.
> 
> This issue is documented in GCC bug 101175:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101175  
> 
> The problematic code path occurs in crush_ln() when:
> - x is incremented from xin  
> - (x & 0x18000) == 0 (condition for the optimization)
> - (x & 0x1FFFF) == 0 (zero argument to __builtin_clz)
> 
> Testing with GCC 11.5.0 confirms that specific input values like 0x7FFFF, 
> 0x9FFFF, 0xBFFFF, 0xDFFFF, 0xFFFFF can trigger this condition, causing
> __builtin_clz(0) to be called with undefined behavior.
> 
> Add a zero check before calling __builtin_clz() to ensure defined behavior
> across all GCC versions and optimization levels.
> 
> Signed-off-by: Huazhao Chen <lyican53@gmail.com>
> ---
> net/ceph/crush/mapper.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/ceph/crush/mapper.c b/net/ceph/crush/mapper.c
> index 1234567..abcdef0 100644
> --- a/net/ceph/crush/mapper.c
> +++ b/net/ceph/crush/mapper.c
> @@ -262,7 +262,8 @@ static __u64 crush_ln(unsigned int xin)
> 	 * do it in one step instead of iteratively
> 	 */
> 	if (!(x & 0x18000)) {
> -		int bits = __builtin_clz(x & 0x1FFFF) - 16;
> +		u32 masked = x & 0x1FFFF;
> +		int bits = masked ? __builtin_clz(masked) - 16 : 16;
> 		x <<= bits;
> 		iexpon = 15 - bits;
> 	}

Let me spend some time for reproduction the issue and testing the patch. I'll be
back ASAP.

Thanks,
Slava.