Re: [PATCH v2 02/20] vmcore: rearrange program headers without assuming consequtive PT_NOTE entries

Zhang Yanfei
2013-03-05T09:37:22Z

于 2013年03月05日 17:02, HATAYAMA Daisuke 写道:
> From: Zhang Yanfei <zhangyanfei@cn.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@jp.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.
>

I see. memmove will always handle well even if there is overlapping between dest and src.

Thanks
Zhang