On Mon, Apr 1, 2024 at 11:24 AM Nathan Chancellor nathan@kernel.org wrote:

After an innocuous optimization change in LLVM main (19.0.0), x86_64 allmodconfig (which enables CONFIG_KCSAN / -fsanitize=thread) fails to build due to the checks in check_copy_size():

In file included from net/bluetooth/sco.c:27: In file included from include/linux/module.h:13: In file included from include/linux/stat.h:19: In file included from include/linux/time.h:60: In file included from include/linux/time32.h:13: In file included from include/linux/timex.h:67: In file included from arch/x86/include/asm/timex.h:6: In file included from arch/x86/include/asm/tsc.h:10: In file included from arch/x86/include/asm/msr.h:15: In file included from include/linux/percpu.h:7: In file included from include/linux/smp.h:118: include/linux/thread_info.h:244:4: error: call to '__bad_copy_from' declared with 'error' attribute: copy source size is too small 244 | __bad_copy_from(); | ^

The same exact error occurs in l2cap_sock.c. The copy_to_user() statements that are failing come from l2cap_sock_getsockopt_old() and sco_sock_getsockopt_old(). This does not occur with GCC with or without KCSAN or Clang without KCSAN enabled.

len is defined as an 'int' because it is assigned from '__user int *optlen'. However, it is clamped against the result of sizeof(), which has a type of 'size_t' ('unsigned long' for 64-bit platforms). This is done with min_t() because min() requires compatible types, which results in both len and the result of sizeof() being casted to 'unsigned int', meaning len changes signs and the result of sizeof() is truncated. From there, len is passed to copy_to_user(), which has a third parameter type of 'unsigned long', so it is widened and changes signs again. This excessive casting in combination with the KCSAN instrumentation causes LLVM to fail to eliminate the __bad_copy_from() call, failing the build.

The official recommendation from LLVM developers is to consistently use long types for all size variables to avoid the unnecessary casting in the first place. Change the type of len to size_t in both l2cap_sock_getsockopt_old() and sco_sock_getsockopt_old(). This clears up the error while allowing min_t() to be replaced with min(), resulting in simpler code with no casts and fewer implicit conversions. While len is a different type than optlen now, it should result in no functional change because the result of sizeof() will clamp all values of optlen in the same manner as before.

Cc: stable@vger.kernel.org Closes: https://github.com/ClangBuiltLinux/linux/issues/2007 Link: https://github.com/llvm/llvm-project/issues/85647 Signed-off-by: Nathan Chancellor nathan@kernel.org

Reviewed-by: Justin Stitt justinstitt@google.com

---

net/bluetooth/l2cap_sock.c | 7 ++++--- net/bluetooth/sco.c | 7 ++++--- 2 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/net/bluetooth/l2cap_sock.c b/net/bluetooth/l2cap_sock.c index 4287aa6cc988..81193427bf05 100644 --- a/net/bluetooth/l2cap_sock.c +++ b/net/bluetooth/l2cap_sock.c @@ -439,7 +439,8 @@ static int l2cap_sock_getsockopt_old(struct socket *sock, int optname, struct l2cap_chan *chan = l2cap_pi(sk)->chan; struct l2cap_options opts; struct l2cap_conninfo cinfo;

•   int len, err = 0;
  

•   int err = 0;
  

•   size_t len;
    u32 opt;
  
    BT_DBG("sk %p", sk);
  

@@ -486,7 +487,7 @@ static int l2cap_sock_getsockopt_old(struct socket *sock, int optname,

            BT_DBG("mode 0x%2.2x", chan->mode);

•           len = min_t(unsigned int, len, sizeof(opts));
  

•           len = min(len, sizeof(opts));
            if (copy_to_user(optval, (char *) &opts, len))
                    err = -EFAULT;
  
  

@@ -536,7 +537,7 @@ static int l2cap_sock_getsockopt_old(struct socket *sock, int optname, cinfo.hci_handle = chan->conn->hcon->handle; memcpy(cinfo.dev_class, chan->conn->hcon->dev_class, 3);

•           len = min_t(unsigned int, len, sizeof(cinfo));
  

•           len = min(len, sizeof(cinfo));
            if (copy_to_user(optval, (char *) &cinfo, len))
                    err = -EFAULT;
  
  

diff --git a/net/bluetooth/sco.c b/net/bluetooth/sco.c index 43daf965a01e..9a72d7f1946c 100644 --- a/net/bluetooth/sco.c +++ b/net/bluetooth/sco.c @@ -967,7 +967,8 @@ static int sco_sock_getsockopt_old(struct socket *sock, int optname, struct sock *sk = sock->sk; struct sco_options opts; struct sco_conninfo cinfo;

•   int len, err = 0;
  

•   int err = 0;
  

•   size_t len;
  
    BT_DBG("sk %p", sk);
  
  

@@ -989,7 +990,7 @@ static int sco_sock_getsockopt_old(struct socket *sock, int optname,

            BT_DBG("mtu %u", opts.mtu);

•           len = min_t(unsigned int, len, sizeof(opts));
  

•           len = min(len, sizeof(opts));
            if (copy_to_user(optval, (char *)&opts, len))
                    err = -EFAULT;
  
  

@@ -1007,7 +1008,7 @@ static int sco_sock_getsockopt_old(struct socket *sock, int optname, cinfo.hci_handle = sco_pi(sk)->conn->hcon->handle; memcpy(cinfo.dev_class, sco_pi(sk)->conn->hcon->dev_class, 3);

•           len = min_t(unsigned int, len, sizeof(cinfo));
  

•           len = min(len, sizeof(cinfo));
            if (copy_to_user(optval, (char *)&cinfo, len))
                    err = -EFAULT;
  
  
  

---

base-commit: 7835fcfd132eb88b87e8eb901f88436f63ab60f7 change-id: 20240401-bluetooth-fix-len-type-getsockopt_old-73c4a8e60444

Best regards,

Nathan Chancellor nathan@kernel.org