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

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Message-ID: <0582476e-415e-3f60-2bb2-6199d0340156@linux.ibm.com>
Date:   Fri, 17 Jul 2020 02:39:14 +0530
From:   Hari Bathini <hbathini@...ux.ibm.com>
To:     Thiago Jung Bauermann <bauerman@...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



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.

Thanks
Hari