linux-kernel - Re: [RFC patch] vsprintf: Add %pav extension for print_vma

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Message-Id: <20150527140406.bcc33ecc79da09ad6782b971@linux-foundation.org>
Date:	Wed, 27 May 2015 14:04:06 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Joe Perches <joe@...ches.com>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC patch] vsprintf: Add %pav extension for print_vma_addr

On Tue, 26 May 2015 16:05:04 -0700 Joe Perches <joe@...ches.com> wrote:

> print_vma_addr is another function to emit useful
> data similar to print_symbol.  The print_symbol
> functionality has been added via %p[fFsS] vsprintf
> extensions.
> 
> Perhaps it's appropriate to add vma_addr address
> decoding to vsprintf too.
> 
> This would allow code conversions where the very
> unlikely interleaving of messages from multiple
> threads might occur.
> 
> like arch/x86/kernel/traps.c:
> 
> from:
>         if (show_unhandled_signals && unhandled_signal(tsk, signr) &&
>             printk_ratelimit()) {
>                 pr_info("%s[%d] trap %s ip:%lx sp:%lx error:%lx",
>                         tsk->comm, tsk->pid, str,
>                         regs->ip, regs->sp, error_code);
>                 print_vma_addr(" in ", regs->ip);
>                 pr_cont("\n");
> to:
>                 pr_info("%s[%d] trap %s ip:%lx sp:%lx error:%lx in %pav\n",
>                         regs->ip, regs->sp, error_code, &regs->ip);
>         }
> 
> Something like:
> 
> ...
>
> @@ -1313,23 +1313,70 @@ char *netdev_feature_string(char *buf, char *end, const u8 *addr,
>  }
>  
>  static noinline_for_stack
> +char *vma_addr(char *buf, char *end, const void *addr,
> +	       struct printf_spec spec, const char *fmt)
> +{
> +	struct mm_struct *mm = current->mm;
> +	struct vm_area_struct *vma;
> +	unsigned long ip = *(unsigned long *)addr;
> +	char *rtn;
> +
> +	/* if we are in atomic contexts (in exception stacks, etc.) */
> +	if (preempt_count())
> +		return string(buf, end, "(atomic context)", spec);

Problems when CONFIG_PREEMPT=n.

> +	down_read(&mm->mmap_sem);
> +	vma = find_vma(mm, ip);
> +	if (vma && vma->vm_file) {
> +		struct file *f = vma->vm_file;
> +		char *gfp_buf = (char *)__get_free_page(GFP_KERNEL);

We shouldn't assume we can use GFP_KERNEL here.  Even if the
preempt_count() worked, we might be in a context which requires
GFP_NOFS or GFP_NOIO.

> +		if (gfp_buf) {
> +			char *p = d_path(&f->f_path, gfp_buf, PAGE_SIZE);
> +
> +			if (IS_ERR(p))
> +				p = "?";
> +
> +			rtn = buf + snprintf(buf, end > buf ? end - buf : 0,
> +					     "%s[%lx+%lx]",
> +					     kbasename(p),
> +					     vma->vm_start,
> +					     vma->vm_end - vma->vm_start);
> +
> +			free_page((unsigned long)gfp_buf);
> +		} else {
> +			rtn = string(buf, end, "(__get_free_page failed)", spec);
> +		}
> +	} else {
> +		rtn = string(buf, end, "(find_vma failed)", spec);
> +	}
> +
> +	up_read(&mm->mmap_sem);
> +
> +	return rtn;
> +}
>
> ...
>
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