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SubjectRe: dangling pointer to '__str' error on ppc64_defconfig, GCC 12.1.0
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Petr Mladek <pmladek@suse.com> writes:
> On Thu 2022-05-26 16:17:15, Bagas Sanjaya wrote:
>> Hi everyone,
>>
>> Attempting to build ppc64_defconfig kernel with powerpc64-unknown-linux-gnu
>> (GCC 12.1.0) on v5.18, I got build error on ftrace.o:
>>
>> CC arch/powerpc/kernel/trace/ftrace.o
>> CC init/init_task.o
>> In file included from ./include/asm-generic/bug.h:22,
>> from ./arch/powerpc/include/asm/bug.h:156,
>> from ./include/linux/bug.h:5,
>> from ./include/linux/thread_info.h:13,
>> from ./include/asm-generic/preempt.h:5,
>> from ./arch/powerpc/include/generated/asm/preempt.h:1,
>> from ./include/linux/preempt.h:78,
>> from ./include/linux/spinlock.h:55,
>> from arch/powerpc/kernel/trace/ftrace.c:16:
>> arch/powerpc/kernel/trace/ftrace.c: In function 'ftrace_modify_code':
>> ./include/linux/printk.h:446:44: error: using a dangling pointer to '__str' [-Werror=dangling-pointer=]
>> 446 | #define printk(fmt, ...) printk_index_wrap(_printk, fmt, ##__VA_ARGS__)
>> | ^
>> ./include/linux/printk.h:418:17: note: in definition of macro 'printk_index_wrap'
>> 418 | _p_func(_fmt, ##__VA_ARGS__); \
>> | ^~~~~~~
>> ./include/linux/printk.h:489:9: note: in expansion of macro 'printk'
>> 489 | printk(KERN_ERR pr_fmt(fmt), ##__VA_ARGS__)
>> | ^~~~~~
>> arch/powerpc/kernel/trace/ftrace.c:75:17: note: in expansion of macro 'pr_err'
>> 75 | pr_err("%p: replaced (%s) != old (%s)",
>> | ^~~~~~
>> In file included from ./arch/powerpc/include/asm/code-patching.h:14,
>> from arch/powerpc/kernel/trace/ftrace.c:26:
>> ./arch/powerpc/include/asm/inst.h:156:14: note: '__str' declared here
>> 156 | char __str[PPC_INST_STR_LEN]; \
>> | ^~~~~
>
> IMHO, the problem is in the macro:
>
> #define ppc_inst_as_str(x) \
> ({ \
> char __str[PPC_INST_STR_LEN]; \
> __ppc_inst_as_str(__str, x); \
> __str; \
> })
>
> The buffer __str is defined inside the code block {} and
> the macro passes the pointer to the buffer outside.
> IMHO, from the compiler POV, the buffer does not exist
> outside of the code block.

This warning seems kind of incompatible with statement expressions. But
I guess the compiler is happy as long as the value returned is not a
pointer. So I'm not sure I agree with the compiler here, but too bad for
me I guess.

This macro has always been a bit awkward, I think I'm inclined to just
get rid of it completely, and print the ulong value. I'll send a patch.

Anyway it's definitely not your problem, sorry you ended up on Cc.

cheers

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