I want to add EWR(Execute/Write/Read) memory attribute to the .data section at compile time.
MSVC compiler can do this by simply adding the "/SECTION:.data,EWR" linker option.
But I don't know how to do this in GCC compiler (MinGW).
Please help!
The simplest way I could think of is adding a dummy file to the link with a .data.* section that has the desired flags. That dummy file is easy to produce using assembly:
.section .data.fake, "axw"
(assuming you are on an ELF platrofm, using gas).
Say, the file containing the above is called dummy.s, then you can either assemble it manually, and then add the result to the link
as dummy.s -o dummy.o
gcc <all your normal .o files> dummy.o # or `ld`
Or just pass the assebmly file to the driver:
gcc <all your normal files> dummy.s
Note that the assembler gives a warning:
Warning: setting incorrect section attributes for .data.fake
because of the unusual flags. It does set them as asked though.
Now, let me explain a little what is going on.
All the input .data* sections are merged into the output .data section. See the default linker script (ld --verbose):
.data :
{
*(.data .data.* .gnu.linkonce.d.*)
SORT(CONSTRUCTORS)
}
The flags of the output section are the union of the flags of the input section (I'm pretty sure this behavior is documented in the linker manual). That is how the trick works. The flags for the segment holding the resulting .data are then also computed as the union of the flags of the sections it contains. See the output of readelf -lW to make sure you got the result you expect. I have
...
LOAD 0x002e28 0x0000000000003e28 0x0000000000003e28 0x000200 0x000208 *RWE* 0x1000
...
05 .init_array .fini_array .dynamic .got .got.plt *.data* .bss
Related
I'm attempting to convert an assembly file to C++ for use as a small and easy to insert "trampoline" loader for another library. It is injected into another program at runtime, then loads a library, runs a function inside of it, and frees it. This is simply to avoid needing multiple lengthy calls to WriteProccessMemory, and to allow certain runtime checks if needed.
Originally, I wrote the code in assembly as it gave me a high degree of control over the structure of the file. I ended up with a ~128 byte file structured as followed:
<Relocation Header> // Table of function pointers filled in by the loading code
<Code>
<Static Data>
The size/structure of the header is known at compile-time, also allowing the entry point to be calculated, so there is very little code needed to load this.
The problem is that sharing the structure of the header between my assembler (NASM) and compiler (GCC) is... difficult, hence the rewrite.
I've come up with this series of commands to compile/link the C++ code:
g++ -c -O3 -fpic Loader.cpp
g++ -O3 -shared -nostdlib Loader.o
Running objcopy -O binary -j .text a.exe then gives a binary file only about 95 bytes in size (I manually inserted some padding in the assembly version to make it clear when debugging where "sections" are).
Only one problem (at least for this question), the variable offsets haven't been relocated (obviously). Viewing the binary, I can see lines like mov rcx, QWORD PTR [rip+0x4fc9]. Clearly, this will not be valid in a 95 byte file. Is there a way (preferably using GCC or a program in Binutils) that I can get a stripped binary with correct offsets? The solution doesn't have to be a post-process like objcopy, it can happen during any part of the build proccess.
I'd really like to avoid any unneeded information in the file, it wouldn't necessarily be detrimental, but this is meant to be super lightweight. The file does not need to be directly runnable (the entry-point does not have to be 0).
Also to be clear, I'm not asking for a simple addition/subtraction to all pointers, GCC's generated addresses are spread across memory, they should be up against the code.
Although incomplete and needing some changes, I think I've come up with a functioning solution for now.
I compile as before, but link with a slightly different command: g++ -T lnkscrpt.txt -O3 -nostdlib Loader.o (-shared just makes the linker complain about missing a DllMain).
lnkscrpt.txt is an ld linker script (https://ftp.gnu.org/old-gnu/Manuals/ld-2.9.1/html_node/ld_5.html#SEC5) as follows:
SECTIONS
{
. = 0x00;
.bss : { *(.bss) }
.text : { *(.text) }
.data : { *(.rdata) *(.data) }
/DISCARD/ : {*(*)}
}
This preserves the order I want and discards any other default sections.
Finally I run objcopy -O binary -j .* --set-section-flags .bss=alloc,load,contents a.exe
to copy over the remaining sections to a flat binary. The --set-section-flags option simply insures that the binary contains space allocated for the .bss section.
This results in a 128 byte binary, laid out in the exact same way as my custom assembly version, using correct offsets, and not containing any unneeded data.
I am writing a simple hello world bootloader in C with inline assembly using this article. Nothing fancy, no kernel loading and other advanced topics. Just a plain old "hello world" message.
Here are my files:
boot.c
/* generate 16-bit code */
__asm__(".code16\n");
/* jump boot code entry */
__asm__("jmpl $0x0000, $main\n");
/* user defined function to print series of characters terminated by null
character */
void printString(const char* pStr) {
while (*pStr) {
__asm__ __volatile__ (
"int $0x10" : : "a"(0x0e00 | *pStr), "b"(0x0007)
);
++pStr;
}
}
void main() {
/* calling the printString function passing string as an argument */
printString("Hello, world!");
}
boot.ld
ENTRY(main);
SECTIONS
{
. = 0x7C00;
.text : AT(0x7C00)
{
*(.text);
}
.sig : AT(0x7DFE)
{
SHORT(0xaa55);
}
}
I then ran the following commands: (different from the first article; adapted from another StackOverflow article as the commands in the first article won't work for me)
gcc -std=c99 -c -g -Os -march=i686 -m32 -ffreestanding -Wall -Werror boot.c -o boot.o
ld -static -T boot.ld -m elf_i386 -nostdlib --nmagic -o boot.elf boot.o
The first line compiles successfully, but I get errors upon executing the second line:
ld: warning: cannot find entry symbol main; defaulting to 0000000000007c00
boot.o:boot.c:(.text+0x2): undefined reference to 'main'
boot.o: In function 'main':
C:(...)/boot.c:16: undefined reference to '__main'
C:(...)/boot.c:16:(.text.startup+0xe): relocation truncated to fit: DISP16 against undefined symbol '__main'
What's wrong? I use Windows 10 x64 with the gcc compiler that comes with Dev-C++.
I'd suggest an i686-elf cross compiler rather than using a native windows compiler and tool chain. I think part of your problem is peculiarities related to the Windows i386pe format.
The .sig section is likely not being written at all since that unknown section probably isn't marked allocatable data. The result of that is the signature isn't written to the final binary file. It is also possible the virtual memory address (VMA) is not being set in boot.ld so it may not advance the boot signature into the last 2 bytes of the 512 byte sector. As well with the Windows format read only data will be placed in sections starting with .rdata. You'll want to make sure those are included after the data section and before the boot signature. Failure to do this will default the linker script into placing unprocessed input sections at the end beyond the boot signature.
Assuming you have made the changes as you mentioned in the comments about the extra underscores your files may work this way:
boot.ld:
ENTRY(__main);
SECTIONS
{
. = 0x7C00;
.text : AT(0x7C00)
{
*(.text);
}
.data :
{
*(.data);
*(.rdata*);
}
.sig 0x7DFE : AT(0x7DFE) SUBALIGN(0)
{
SHORT(0xaa55);
}
}
The commands to compile/link and adjust the .sig section to be a regular readonly allocated data section would look like:
gcc.exe -std=c99 -c -g -Os -march=i686 -m32 -ffreestanding -Wall -Werror boot.c -o boot.o
ld.exe -mi386pe -static -T boot.ld -nostdlib --nmagic -o boot.elf boot.o
# This adjusts the .sig section attributes and updates boot.elf
objcopy --set-section-flags .sig=alloc,contents,load,data,readonly boot.elf boot.elf
# Convert to binary
objcopy -O binary boot.elf boot.bin
Other Observations
Your use of __asm__(".code16\n"); will not generate usable code for a bootloader. You'll want to use the experimental pseudo 16-bit code generation that forces the assembler to modify instructions to be compatible with 32-bit code but encoded to be usable in 16-bit real mode. You can do this by using __asm__(".code16gcc\n"); at the top of each C/C++ files.
This tutorial has some bad advice. The global level basic assembly statement that does the JMP to main may be relocated to somewhere other than the beginning of the bootloader (some optimization levels may cause this). The startup code doesn't set ES, DS, CS to 0x0000, nor does it set the SS:SP stack segment and pointer. This can cause problems.
If trying to run from a USB drive on real hardware you may find you'll need a Boot Parameter Block. This Stackoverflow Answer I wrote discusses this issue and a possible work around under Real Hardware / USB / Laptop Issues
Note: The only useful code that GCC currently generates is 32-bit code that can run in 16-bit real mode. This means that you can't expect this code to run on a processor earlier than a 386 like the 80186/80286/8086 etc.
My general recommendation is to not create bootloaders with GCC unless you know what you are really doing and understand all the nuances involved. Writing it in assembly is probably a much better idea.
If you want a C/C++ compiler that generates true 16-bit code you may wish to look at OpenWatcom
I would like to load .symtab into memory with gdb debugger.
At most two steps are required for a normal section (for some section, e.g. .text, .data, ... , step 1 can be skipped cause is automatically set by ld):
1 - Set the Alloc flag (in case of a special section) to the section in the ELF. This can be done in this way for a normal section.
arm-none-eabi-objcopy --set-section-flags .sectionName=alloc src.elf dst.elf
2 - Set the address to the section. This can be done in 2 ways for a normal section AFAIK
A - Specifying the section memory area in the LD script e.g. for text section:
.text :
{
*(.text)
*(.text*)
} > FLASH
B - Using again objcopy
arm-none-eabi-objcopy --change-section-address .sectioName=0x0ABCD src.elf dst.elf
since .symtab is generated automatically by the linker I cannot treat it as a normal section so none of the steps above works.
Does anyone have any idea on how to solve this?
I already successfully implemented a workaround that to generate a new elf stripping all unneeded sections, and this works but then you have to load two elfs and i'm looking for a cleaner solution.
I know there are some other similar questions about this out there, be it StackOverflow or not. I've researched a lot for this, and still didn't find a single solution.
I'm doing an operative system as a side project. I've been doing all in Assembly, but now I wanna join C code.
To test, I made this assembly code file (called test.asm):
[BITS 32]
GLOBAL _a
SECTION .text
_a:
jmp $
Then I made this C file (called main.c):
extern void a(void);
int main(void)
{
a();
}
To link, I used this file (called make.bat):
"C:\minGW\bin\gcc.exe" -ffreestanding -c -o c.o main.c
nasm -f coff -o asm.o test.asm
"C:\minGW\bin\ld.exe" -Ttext 0x100000 --oformat binary -o out.bin c.o asm.o
pause
I've been researching for ages, and I'm still struggling to find an answer. I hope this won't be flagged as duplicate. I acknowledge about the existence of similar questions, but all have different answers, and none work for me.
Question: What am I doing wrong?
Old MinGW versions had the problem that "ld" was not able to create non-PE files at all.
Maybe current versions have the same problem.
The work-around was creating a PE file with "ld" and then to transform the PE file to binary, HEX or S19 using "objcopy".
--- EDIT ---
Thinking about the question again I see two problems:
As I already said some versions of "ld" have problems creating "binary" output (instead of "PE", "ELF" or whatever format is used).
Instead of:
ld.exe --oformat binary -o file.bin c.o asm.o
You should use the following sequence to create the binary file:
ld.exe -o file.tmp c.o asm.o
objcopy -O binary file.tmp file.bin
This will create an ".exe" file named "binary.tmp"; then "objcopy" will create the raw data from the ".exe" file.
The second problem is the linking itself:
"ld" assumes a ".exe"-like file format - even if the output file is a binary file. This means that ...
... you cannot even be sure if the object code of "main.o" is really placed at the first address of the resulting object code. "ld" would also be allowed to put the code of "a()" before "main()" or even put "internal" code before "a()" and "main()".
... addressing works a bit differently which means that a lot of padding bytes will be created (maybe at the start of the file!) if you do something wrong.
The only possibility I see is to create a "linker script" (sometimes called "linker command file") and to create a special section in the assembler code (because I normally use another assembler than "nasm" I do not know if the syntax here is correct):
[BITS 32]
GLOBAL _a
SECTION .entry
jmp _main
SECTION .text
_a:
jmp $
In the linker script you can specify which sections appear in which order. Specify that ".entry" is the first section of the file so you can be sure it is the first instruction of the file.
In the linker script you may also say that multiple sections (e.g. ".entry", ".text" and ".data") should be combined into a single section. This is useful because sections are normally 0x1000-byte-aligned in PE files! If you do not combine multiple sections into one you'll get a lot of stub bytes between the sections!
Unfortunately I'm not the expert for linker scripts so I cannot help you too much with that.
Using "-Ttext" is also problematic:
In PE files the actual address of a section is calculated as "image base" + "relative address". The "-Ttext" argument will influence the "relative address" only. Because the "relative address" of the first section is typically fixed to 0x1000 in Windows a "-Ttext 0x2000" would do nothing but filling 0x1000 stub bytes at the start of the first section. However you do not influence the start address of ".text" at all - you only fill stub bytes at the start of the ".text" section so that the first useful byte is located at 0x2000. (Maybe some "ld" versions behave differently.)
If you wish that the first section of your file is located at address 0x100000 you should use the equivalent of "-Ttext 0x1000" in the linker script (-Ttext is not used if a linker script is used) and define the "image base" to 0xFF000:
ld.exe -T linkerScript.ld --image-base 0xFF000 -o binary.tmp a.o main.o
The memory address of the ".text" section will be 0xFF000 + 0x1000 = 0x100000.
(And the first byte of the binary file generated by "objcopy" will be the first byte of the first section - representing memory address 0x100000.)
As I'm new to binutils, gcc ant others, I have some general questions, anwsers on which I havn't found in manuals.
I'm using C and assembly(nasm syntax) and I need raw binary files on output. First of all, I compile my code to objec file with parameters:
cc -nostartfiles -nostdlib -c -ffreestanding <input file(s)> ;cc or gcc no matter
Then I link all the files using simple script which only puts segments in needed order.
ld -T <script> -o <o.file> <in.file(s)> ;nothing special here
And to get raw binary I use objcopy
objcopy -O binary <o.file> <in.file> ;can't be simplier
All in all, I need binary file only with .text and .data segments in it and 32-bit code.
1.Can i get this way what I want?
2.Are there other ways to do that? (no matter easier or more complicated)
Thank you for help.
I haven't problems compiling Asm code, almost all problems with C code.
Once I came across a ld manual page and /DISCARD/ block was said to exclude everything listed in it from final output.
So I've inserted this block after the .text, .data and .bss blocks
/DISCARD/ :
{
*(.comment)
*(.eh_frame)
*(.note.GNU-stack)
}
As well as this line in the very beginning of my linker script.
OUTPUT_FORMAT("binary")
Therefore, I do not need to use objcopy anymore.
You need to compile the source files using this command
nasm -o bin <SOURCE FILES>
This will produce pure binary output.