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Kernel link script parsing
2022-07-06 17:39:00 【Wzzzzzzx】
The original text can be found in Kernel link script parsing Check out , Subsequent updates will be updated to Personal blog
The specific code can be seen in Warehouse , Here, choose a few to talk about .
Output architecture
OUTPUT_ARCH
This command is used to specify the system architecture of the output file , What we use here is riscv.
entry point
ENTRY
Defines the entry point of the program ,xv6 The default entry point for is _entry
. The entry point code is entry.S Inside .
Position counter
Get into SECTIONS
after , The default initial address of the current program is 0x0. Here with Linux The file system of , use “.” To represent the current address . The name of this symbol is Position counter (location counter), Indicates the following paragraph / The memory address corresponding to the variable . This means that if you give it directly “.” assignment , It is equivalent to switching the working directory , The relative position of subsequent segments will change .
Here is a simple example
SECTIONS
{
. = 0x10000;
.text : { *(.text) }
. = 0x8000000;
.data : { *(.data) }
.bss : { *(.bss) }
}
Get into SECTIONS
after , The position counter will be set to 0x10000, So the linker will .text
The address of is set to 0x10000. complete .text
After setting , The position counter will be changed to 0x8000000,.data
The address of will be set to 0x8000000. turn .bss
When , It will follow .data
paragraph , At this time, the value of the position counter is 0x8000000 add .data
The size of the segment .
xv6
Initial address setting
As soon as you enter SECTIONS
The script will set the current initial path to 0x80000000. The teacher said in class that this address is qemu Approved address , The first instruction that must be placed is it . Book 2.6 section The saying is 0x0 To 0x80000000 This address is used to place IO equipment , So the address of the first instruction must be 0x80000000.
Segment content parsing
.text : {
*(.text .text.*)
. = ALIGN(0x1000);
_trampoline = .;
*(trampsec)
. = ALIGN(0x1000);
ASSERT(. - _trampoline == 0x1000, "error: trampoline larger than one page");
PROVIDE(etext = .);
}
The segment settings of the kernel are similar , So look at one .text
That's all right. .
.text
The first expression of the segment *(.text .text.*)
, Among them *
The wildcard . It represents .text
and .text.*
Segments are placed in the output file .text
In the paragraph .
Then there are orders ALIGN(exp)
, This command will return the position counter to align to the next exp
Address of boundary , It does not modify the value of the position counter .
PROVIDE
This order is similar to GCC Medium attribute((weak))
. Now we only need to understand it simply as defining a symbol .
remainder _trampoline
and trampsec
The chapter of the page table will cover , Then you can go and understand .
Reference resources
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