On Tue, Jul 15, 2025 at 10:24:50PM -0700, Nathan Chancellor wrote:
On Wed, Jul 16, 2025 at 07:06:50AM +0200, Greg Kroah-Hartman wrote:
On Tue, Jul 15, 2025 at 01:33:32PM -0700, Nathan Chancellor wrote:
After a recent change in clang to expose uninitialized warnings from const variables [1], there is a warning in cxacru_heavy_init():
drivers/usb/atm/cxacru.c:1104:6: error: variable 'bp' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized] 1104 | if (instance->modem_type->boot_rom_patch) { | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ drivers/usb/atm/cxacru.c:1113:39: note: uninitialized use occurs here 1113 | cxacru_upload_firmware(instance, fw, bp); | ^~ drivers/usb/atm/cxacru.c:1104:2: note: remove the 'if' if its condition is always true 1104 | if (instance->modem_type->boot_rom_patch) { | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ drivers/usb/atm/cxacru.c:1095:32: note: initialize the variable 'bp' to silence this warning 1095 | const struct firmware *fw, *bp; | ^ | = NULL
This warning occurs in clang's frontend before inlining occurs, so it cannot notice that bp is only used within cxacru_upload_firmware() under the same condition that initializes it in cxacru_heavy_init(). Just initialize bp to NULL to silence the warning without functionally changing the code, which is what happens with modern compilers when they support '-ftrivial-auto-var-init=zero' (CONFIG_INIT_STACK_ALL_ZERO=y).
We generally do not want to paper over compiler bugs, when our code is correct, so why should we do that here? Why not fix clang instead?
I would not really call this a compiler bug. It IS passed uninitialized to this function and while the uninitialized value is not actually used, clang has no way of knowing that at this point in its pipeline, so I don't think warning in this case is unreasonable. This type of warning is off for GCC because of how unreliable it was when it is done in the middle end with optimizations. Furthermore, it is my understanding based on [1] that just the passing of an uninitialized variable in this manner is UB.
Ah, I see now what you are referring to, sorry. I'll go queue this up now, thanks.
greg k-h