linux-kernel - Re: [PATCH v3 04/12] ppc64/kexec_file: avoid stomping memory used by special regions

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Date:   Thu, 16 Jul 2020 18:59:49 -0300
From:   Thiago Jung Bauermann <bauerman@...ux.ibm.com>
To:     Hari Bathini <hbathini@...ux.ibm.com>
Cc:     Pingfan Liu <piliu@...hat.com>, Nayna Jain <nayna@...ux.ibm.com>,
        Kexec-ml <kexec@...ts.infradead.org>,
        Mahesh J Salgaonkar <mahesh@...ux.ibm.com>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        lkml <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...abs.org>,
        Sourabh Jain <sourabhjain@...ux.ibm.com>,
        Petr Tesarik <ptesarik@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Young <dyoung@...hat.com>,
        Vivek Goyal <vgoyal@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>
Subject: Re: [PATCH v3 04/12] ppc64/kexec_file: avoid stomping memory used by special regions


Hari Bathini <hbathini@...ux.ibm.com> writes:

> On 15/07/20 8:09 am, Thiago Jung Bauermann wrote:
>> 
>> Hari Bathini <hbathini@...ux.ibm.com> writes:
>> 
>
> <snip>
>  
>>> +/**
>>> + * __locate_mem_hole_top_down - Looks top down for a large enough memory hole
>>> + *                              in the memory regions between buf_min & buf_max
>>> + *                              for the buffer. If found, sets kbuf->mem.
>>> + * @kbuf:                       Buffer contents and memory parameters.
>>> + * @buf_min:                    Minimum address for the buffer.
>>> + * @buf_max:                    Maximum address for the buffer.
>>> + *
>>> + * Returns 0 on success, negative errno on error.
>>> + */
>>> +static int __locate_mem_hole_top_down(struct kexec_buf *kbuf,
>>> +				      u64 buf_min, u64 buf_max)
>>> +{
>>> +	int ret = -EADDRNOTAVAIL;
>>> +	phys_addr_t start, end;
>>> +	u64 i;
>>> +
>>> +	for_each_mem_range_rev(i, &memblock.memory, NULL, NUMA_NO_NODE,
>>> +			       MEMBLOCK_NONE, &start, &end, NULL) {
>>> +		if (start > buf_max)
>>> +			continue;
>>> +
>>> +		/* Memory hole not found */
>>> +		if (end < buf_min)
>>> +			break;
>>> +
>>> +		/* Adjust memory region based on the given range */
>>> +		if (start < buf_min)
>>> +			start = buf_min;
>>> +		if (end > buf_max)
>>> +			end = buf_max;
>>> +
>>> +		start = ALIGN(start, kbuf->buf_align);
>>> +		if (start < end && (end - start + 1) >= kbuf->memsz) {
>> 
>> This is why I dislike using start and end to express address ranges:
>> 
>> While struct resource seems to use the [address, end] convention, my
>
> struct crash_mem also uses [address, end] convention.
> This off-by-one error did not cause any issues as the hole start and size we try to find
> are at least page aligned.
>
> Nonetheless, I think fixing 'end' early in the loop with "end -= 1" would ensure
> correctness while continuing to use the same convention for structs crash_mem & resource.

Sounds good.

-- 
Thiago Jung Bauermann
IBM Linux Technology Center