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Subject: Re: [PATCH] powerpc: mark local variables around longjmp as volatile
To:     Arnd Bergmann <arnd@kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Nicholas Piggin <npiggin@gmail.com>,
        Ravi Bangoria <ravi.bangoria@linux.ibm.com>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
        Linux Doc Mailing List <linux-doc@vger.kernel.org>
References: <20210429080708.1520360-1-arnd@kernel.org>
From:   Christophe Leroy <christophe.leroy@csgroup.eu>
Message-ID: <5e3a48a2-3ce2-fadb-ee3d-4241bacf618f@csgroup.eu>
Date:   Fri, 25 Jun 2021 12:35:11 +0200
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Le 29/04/2021 à 10:06, Arnd Bergmann a écrit :
> From: Arnd Bergmann <arnd@arndb.de>
> 
> gcc-11 points out that modifying local variables next to a
> longjmp/setjmp may cause undefined behavior:
> 
> arch/powerpc/kexec/crash.c: In function 'crash_kexec_prepare_cpus.constprop':
> arch/powerpc/kexec/crash.c:108:22: error: variable 'ncpus' might be clobbered by 'longjmp' or 'vfork' [-Werror=clobbere
> d]
> arch/powerpc/kexec/crash.c:109:13: error: variable 'tries' might be clobbered by 'longjmp' or 'vfork' [-Werror=clobbere
> d]
> arch/powerpc/xmon/xmon.c: In function 'xmon_print_symbol':
> arch/powerpc/xmon/xmon.c:3625:21: error: variable 'name' might be clobbered by 'longjmp' or 'vfork' [-Werror=clobbered]
> arch/powerpc/xmon/xmon.c: In function 'stop_spus':
> arch/powerpc/xmon/xmon.c:4057:13: error: variable 'i' might be clobbered by 'longjmp' or 'vfork' [-Werror=clobbered]
> arch/powerpc/xmon/xmon.c: In function 'restart_spus':
> arch/powerpc/xmon/xmon.c:4098:13: error: variable 'i' might be clobbered by 'longjmp' or 'vfork' [-Werror=clobbered]
> arch/powerpc/xmon/xmon.c: In function 'dump_opal_msglog':
> arch/powerpc/xmon/xmon.c:3008:16: error: variable 'pos' might be clobbered by 'longjmp' or 'vfork' [-Werror=clobbered]
> arch/powerpc/xmon/xmon.c: In function 'show_pte':
> arch/powerpc/xmon/xmon.c:3207:29: error: variable 'tsk' might be clobbered by 'longjmp' or 'vfork' [-Werror=clobbered]
> arch/powerpc/xmon/xmon.c: In function 'show_tasks':
> arch/powerpc/xmon/xmon.c:3302:29: error: variable 'tsk' might be clobbered by 'longjmp' or 'vfork' [-Werror=clobbered]
> arch/powerpc/xmon/xmon.c: In function 'xmon_core':
> arch/powerpc/xmon/xmon.c:494:13: error: variable 'cmd' might be clobbered by 'longjmp' or 'vfork' [-Werror=clobbered]
> arch/powerpc/xmon/xmon.c:860:21: error: variable 'bp' might be clobbered by 'longjmp' or 'vfork' [-Werror=clobbered]
> arch/powerpc/xmon/xmon.c:860:21: error: variable 'bp' might be clobbered by 'longjmp' or 'vfork' [-Werror=clobbered]
> arch/powerpc/xmon/xmon.c:492:48: error: argument 'fromipi' might be clobbered by 'longjmp' or 'vfork' [-Werror=clobbered]
> 
> According to the documentation, marking these as 'volatile' is
> sufficient to avoid the problem, and it shuts up the warning.


I think this change deserves some comment in the code, and maybe also an update of 
https://www.kernel.org/doc/html/latest/process/volatile-considered-harmful.html

Otherwise, there's a risk that one day or another, someone removes those 'volatile' markings.

Christophe


> 
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>   arch/powerpc/kexec/crash.c |  4 ++--
>   arch/powerpc/xmon/xmon.c   | 22 +++++++++++-----------
>   2 files changed, 13 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/powerpc/kexec/crash.c b/arch/powerpc/kexec/crash.c
> index 0196d0c211ac..10f997e6bb95 100644
> --- a/arch/powerpc/kexec/crash.c
> +++ b/arch/powerpc/kexec/crash.c
> @@ -105,8 +105,8 @@ void crash_ipi_callback(struct pt_regs *regs)
>   static void crash_kexec_prepare_cpus(int cpu)
>   {
>   	unsigned int msecs;
> -	unsigned int ncpus = num_online_cpus() - 1;/* Excluding the panic cpu */
> -	int tries = 0;
> +	volatile unsigned int ncpus = num_online_cpus() - 1;/* Excluding the panic cpu */
> +	volatile int tries = 0;
>   	int (*old_handler)(struct pt_regs *regs);
>   
>   	printk(KERN_EMERG "Sending IPI to other CPUs\n");
> diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
> index c8173e92f19d..ce0eacf77645 100644
> --- a/arch/powerpc/xmon/xmon.c
> +++ b/arch/powerpc/xmon/xmon.c
> @@ -489,10 +489,10 @@ static void xmon_touch_watchdogs(void)
>   	touch_nmi_watchdog();
>   }
>   
> -static int xmon_core(struct pt_regs *regs, int fromipi)
> +static int xmon_core(struct pt_regs *regs, volatile int fromipi)
>   {
> -	int cmd = 0;
> -	struct bpt *bp;
> +	volatile int cmd = 0;
> +	struct bpt *volatile bp;
>   	long recurse_jmp[JMP_BUF_LEN];
>   	bool locked_down;
>   	unsigned long offset;
> @@ -857,7 +857,7 @@ static inline void force_enable_xmon(void)
>   static struct bpt *at_breakpoint(unsigned long pc)
>   {
>   	int i;
> -	struct bpt *bp;
> +	struct bpt *volatile bp;
>   
>   	bp = bpts;
>   	for (i = 0; i < NBPTS; ++i, ++bp)
> @@ -3005,7 +3005,7 @@ static void dump_opal_msglog(void)
>   {
>   	unsigned char buf[128];
>   	ssize_t res;
> -	loff_t pos = 0;
> +	volatile loff_t pos = 0;
>   
>   	if (!firmware_has_feature(FW_FEATURE_OPAL)) {
>   		printf("Machine is not running OPAL firmware.\n");
> @@ -3160,7 +3160,7 @@ memzcan(void)
>   		printf("%.8lx\n", a - mskip);
>   }
>   
> -static void show_task(struct task_struct *tsk)
> +static void show_task(struct task_struct *volatile tsk)
>   {
>   	char state;
>   
> @@ -3204,7 +3204,7 @@ static void format_pte(void *ptep, unsigned long pte)
>   static void show_pte(unsigned long addr)
>   {
>   	unsigned long tskv = 0;
> -	struct task_struct *tsk = NULL;
> +	struct task_struct *volatile tsk = NULL;
>   	struct mm_struct *mm;
>   	pgd_t *pgdp;
>   	p4d_t *p4dp;
> @@ -3299,7 +3299,7 @@ static void show_pte(unsigned long addr)
>   static void show_tasks(void)
>   {
>   	unsigned long tskv;
> -	struct task_struct *tsk = NULL;
> +	struct task_struct *volatile tsk = NULL;
>   
>   	printf("     task_struct     ->thread.ksp    ->thread.regs    PID   PPID S  P CMD\n");
>   
> @@ -3622,7 +3622,7 @@ static void xmon_print_symbol(unsigned long address, const char *mid,
>   			      const char *after)
>   {
>   	char *modname;
> -	const char *name = NULL;
> +	const char *volatile name = NULL;
>   	unsigned long offset, size;
>   
>   	printf(REG, address);
> @@ -4054,7 +4054,7 @@ void xmon_register_spus(struct list_head *list)
>   static void stop_spus(void)
>   {
>   	struct spu *spu;
> -	int i;
> +	volatile int i;
>   	u64 tmp;
>   
>   	for (i = 0; i < XMON_NUM_SPUS; i++) {
> @@ -4095,7 +4095,7 @@ static void stop_spus(void)
>   static void restart_spus(void)
>   {
>   	struct spu *spu;
> -	int i;
> +	volatile int i;
>   
>   	for (i = 0; i < XMON_NUM_SPUS; i++) {
>   		if (!spu_info[i].spu)
>