Patching ELF with minimum number of change in bytes
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Problem Definition :
I need to patch an elf file. This elf file is produced from the available source code that I have developed. I also have a linker file which is a standard example for my target.
Challenge is to keep changes (byte shifts) in the elf file as minimum when I add a new code line. When I take difference in bytes between original and patched elf files, the first occurrence of byte shift (cause of new code line) is desired to be near end of the elf file.
So adding a new text and data section for patch purpose is a must!
But these sections also be located at the end of elf file.
Here is my test combinations :
What works;
When I add a new functionality to any function, I will rewrite this function in a new source file. And add this function to a new section called patchText (defined in linker file) or whatever. Finally, I change function call instruction manually, it works! But it is manual :( Is there any way to do this automatically?
What does not work;
1) __attribute_((weak)) option -> When I put my patch code after .bss or .data section, linker creates two executable PT_LOAD segments. And When I run the program, it does not work! Because my patch function symbol cannot be resolved.
If I use "weak" attribute without adding new section it works.
So is there any explanation or comment about that? What will be wrong ? or is it possible ?
2) -Wl,-wrap,symboname option : Same problem as in "weak".
Thank you.
c patch elf
asked Nov 22 at 12:36
vilidigonzales
43
add a comment |
up vote
0
down vote
favorite
Problem Definition :
I need to patch an elf file. This elf file is produced from the available source code that I have developed. I also have a linker file which is a standard example for my target.
Challenge is to keep changes (byte shifts) in the elf file as minimum when I add a new code line. When I take difference in bytes between original and patched elf files, the first occurrence of byte shift (cause of new code line) is desired to be near end of the elf file.
So adding a new text and data section for patch purpose is a must!
But these sections also be located at the end of elf file.
Here is my test combinations :
What works;
When I add a new functionality to any function, I will rewrite this function in a new source file. And add this function to a new section called patchText (defined in linker file) or whatever. Finally, I change function call instruction manually, it works! But it is manual :( Is there any way to do this automatically?
What does not work;
1) __attribute_((weak)) option -> When I put my patch code after .bss or .data section, linker creates two executable PT_LOAD segments. And When I run the program, it does not work! Because my patch function symbol cannot be resolved.
If I use "weak" attribute without adding new section it works.
So is there any explanation or comment about that? What will be wrong ? or is it possible ?
2) -Wl,-wrap,symboname option : Same problem as in "weak".
Thank you.
c patch elf
asked Nov 22 at 12:36
vilidigonzales
43
"is there any other way to achieve this" -- probably. But you didn't tell us what you are actually trying to achieve. See xyproblem.info
– Employed Russian
Nov 22 at 17:15
Thank you @EmployedRussian. I have updated my question.
– vilidigonzales
Dec 11 at 14:19
add a comment |
up vote
0
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favorite
up vote
0
down vote
favorite
Problem Definition :
I need to patch an elf file. This elf file is produced from the available source code that I have developed. I also have a linker file which is a standard example for my target.
Challenge is to keep changes (byte shifts) in the elf file as minimum when I add a new code line. When I take difference in bytes between original and patched elf files, the first occurrence of byte shift (cause of new code line) is desired to be near end of the elf file.
So adding a new text and data section for patch purpose is a must!
But these sections also be located at the end of elf file.
Here is my test combinations :
What works;
When I add a new functionality to any function, I will rewrite this function in a new source file. And add this function to a new section called patchText (defined in linker file) or whatever. Finally, I change function call instruction manually, it works! But it is manual :( Is there any way to do this automatically?
What does not work;
1) __attribute_((weak)) option -> When I put my patch code after .bss or .data section, linker creates two executable PT_LOAD segments. And When I run the program, it does not work! Because my patch function symbol cannot be resolved.
If I use "weak" attribute without adding new section it works.
So is there any explanation or comment about that? What will be wrong ? or is it possible ?
2) -Wl,-wrap,symboname option : Same problem as in "weak".
Thank you.
c patch elf
asked Nov 22 at 12:36
vilidigonzales
43
Problem Definition :
I need to patch an elf file. This elf file is produced from the available source code that I have developed. I also have a linker file which is a standard example for my target.
Challenge is to keep changes (byte shifts) in the elf file as minimum when I add a new code line. When I take difference in bytes between original and patched elf files, the first occurrence of byte shift (cause of new code line) is desired to be near end of the elf file.
So adding a new text and data section for patch purpose is a must!
But these sections also be located at the end of elf file.
Here is my test combinations :
What works;
When I add a new functionality to any function, I will rewrite this function in a new source file. And add this function to a new section called patchText (defined in linker file) or whatever. Finally, I change function call instruction manually, it works! But it is manual :( Is there any way to do this automatically?
What does not work;
1) __attribute_((weak)) option -> When I put my patch code after .bss or .data section, linker creates two executable PT_LOAD segments. And When I run the program, it does not work! Because my patch function symbol cannot be resolved.
If I use "weak" attribute without adding new section it works.
So is there any explanation or comment about that? What will be wrong ? or is it possible ?
2) -Wl,-wrap,symboname option : Same problem as in "weak".
Thank you.
c patch elf
c patch elf
asked Nov 22 at 12:36
vilidigonzales
43
asked Nov 22 at 12:36
vilidigonzales
43
edited Dec 11 at 14:15
asked Nov 22 at 12:36
vilidigonzales
43
asked Nov 22 at 12:36
vilidigonzales
43
asked Nov 22 at 12:36
vilidigonzales
43
43
"is there any other way to achieve this" -- probably. But you didn't tell us what you are actually trying to achieve. See xyproblem.info
– Employed Russian
Nov 22 at 17:15
Thank you @EmployedRussian. I have updated my question.
– vilidigonzales
Dec 11 at 14:19
add a comment |
"is there any other way to achieve this" -- probably. But you didn't tell us what you are actually trying to achieve. See xyproblem.info
– Employed Russian
Nov 22 at 17:15
Thank you @EmployedRussian. I have updated my question.
– vilidigonzales
Dec 11 at 14:19
"is there any other way to achieve this" -- probably. But you didn't tell us what you are actually trying to achieve. See xyproblem.info
– Employed Russian
Nov 22 at 17:15
"is there any other way to achieve this" -- probably. But you didn't tell us what you are actually trying to achieve. See xyproblem.info
– Employed Russian
Nov 22 at 17:15
Thank you @EmployedRussian. I have updated my question.
– vilidigonzales
Dec 11 at 14:19
Thank you @EmployedRussian. I have updated my question.
– vilidigonzales
Dec 11 at 14:19
add a comment |
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"is there any other way to achieve this" -- probably. But you didn't tell us what you are actually trying to achieve. See xyproblem.info
– Employed Russian
Nov 22 at 17:15
Thank you @EmployedRussian. I have updated my question.
– vilidigonzales
Dec 11 at 14:19