linux-kernel - Re: [PATCH v2 02/20] vmcore: rearrange program headers without assuming consequtive PT

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Message-Id: <20130305.180243.45653001.d.hatayama@jp.fujitsu.com>
Date:	Tue, 05 Mar 2013 18:02:43 +0900 (JST)
From:	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>
To:	zhangyanfei@...fujitsu.com
Cc:	kexec@...ts.infradead.org, heiko.carstens@...ibm.com,
	linux-kernel@...r.kernel.org, lisa.mitchell@...com,
	kumagai-atsushi@....nes.nec.co.jp, ebiederm@...ssion.com,
	akpm@...ux-foundation.org, cpw@....com, vgoyal@...hat.com
Subject: Re: [PATCH v2 02/20] vmcore: rearrange program headers without
 assuming consequtive PT_NOTE entries

From: Zhang Yanfei <zhangyanfei@...fujitsu.com>
Subject: Re: [PATCH v2 02/20] vmcore: rearrange program headers without assuming consequtive PT_NOTE entries
Date: Tue, 5 Mar 2013 16:36:53 +0800

> 于 2013年03月02日 16:35, HATAYAMA Daisuke 写道:
>> Current code assumes all PT_NOTE headers are placed at the beginning
>> of program header table and they are consequtive. But the assumption
>> could be broken by future changes on either kexec-tools or the 1st
>> kernel. This patch removes the assumption and rearranges program
>> headers as the following conditions are satisfied:
>> 
>> - PT_NOTE entry is unique at the first entry,
>> 
>> - the order of program headers are unchanged during this
>>   rearrangement, only their positions are changed in positive
>>   direction.
>> 
>> - unused part that occurs in the bottom of program headers are filled
>>   with 0.
>> 
>> Also, this patch adds one exceptional case where the number of PT_NOTE
>> entries is somehow 0. Then, immediately go out of the function.
>> 
>> Signed-off-by: HATAYAMA Daisuke <d.hatayama@...fujitsu.com>
>> ---
>> 
>>  fs/proc/vmcore.c |   92 +++++++++++++++++++++++++++++++++++++++++++-----------
>>  1 files changed, 74 insertions(+), 18 deletions(-)
>> 
>> diff --git a/fs/proc/vmcore.c b/fs/proc/vmcore.c
>> index abf4f01..b5c9e33 100644
>> --- a/fs/proc/vmcore.c
>> +++ b/fs/proc/vmcore.c
>> @@ -251,8 +251,7 @@ static u64 __init get_vmcore_size_elf32(char *elfptr)
>>  static int __init merge_note_headers_elf64(char *elfptr, size_t *elfsz,
>>  						struct list_head *vc_list)
>>  {
>> -	int i, nr_ptnote=0, rc=0;
>> -	char *tmp;
>> +	int i, j, nr_ptnote=0, i_ptnote, rc=0;
> 
> After applying the patch, there are two "j" defined.
> 
> 251 static int __init merge_note_headers_elf64(char *elfptr, size_t *elfsz,
> 252                                                 struct list_head *vc_list)
> 253 {
> 254         int i, j, nr_ptnote=0, i_ptnote, rc=0;
> 255         Elf64_Ehdr *ehdr_ptr;
> 256         Elf64_Phdr phdr, *phdr_ptr;
> 257         Elf64_Nhdr *nhdr_ptr;
> 258         u64 phdr_sz = 0, note_off;
> 259 
> 260         ehdr_ptr = (Elf64_Ehdr *)elfptr;
> 261         phdr_ptr = (Elf64_Phdr*)(elfptr + ehdr_ptr->e_phoff);
> 262         for (i = 0; i < ehdr_ptr->e_phnum; i++, phdr_ptr++) {
> 263                 int j;
> 264                 void *notes_section;
> 265                 struct vmcore *new;
> 
> 
> line 254 and 263.
> 

I've already noticed the name of the inner j is never best in meaning
under development but I didn't make patch for it; it's beyond the
scope of this patch series.

I'll replace the outer j by another incremental variable like k. 

> 
>>  	Elf64_Ehdr *ehdr_ptr;
>>  	Elf64_Phdr phdr, *phdr_ptr;
>>  	Elf64_Nhdr *nhdr_ptr;
>> @@ -302,6 +301,39 @@ static int __init merge_note_headers_elf64(char *elfptr, size_t *elfsz,
>>  		kfree(notes_section);
>>  	}
>>  
>> +	if (nr_ptnote == 0)
>> +		goto out;
>> +
>> +	phdr_ptr = (Elf64_Phdr *)(elfptr + ehdr_ptr->e_phoff);
>> +
>> +	/* Remove unwanted PT_NOTE program headers. */
>> +
>> +        /* - 1st pass shifts non-PT_NOTE entries until the first
>> +	     PT_NOTE entry. */
>> +	i_ptnote = -1;
>> +	for (i = 0; i < ehdr_ptr->e_phnum; ++i) {
>> +		if (phdr_ptr[i].p_type == PT_NOTE) {
>> +			i_ptnote = i;
>> +			break;
>> +		}
>> +	}
>> +	BUG_ON(i_ptnote == -1); /* impossible case since nr_ptnote > 0. */
>> +	memmove(phdr_ptr + 1, phdr_ptr,	i_ptnote * sizeof(Elf64_Phdr));
> 
> is there any problem with this move? What is the batch bytes for every loop
> of memmove? 
> 
> if i_ptnode == 2, so we have
> 
> -------------------------------------
> | PT_LOAD 1 | PT_LOAD 2 | PT_NOTE 1 |
> -------------------------------------
> 
> -->
> 
> -------------------------------------
> |           | PT_LOAD 1 | PT_LOAD 2 |
> -------------------------------------
> 
> right? In the move, Does PT_LOAD 1 overwrite the original PT_LOAD 2?
> 

Right and yes, see man memmove and man memcpy, and please compare
them.

Thanks.
HATAYAMA, Daisuke

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