Keep rodata located with the function that created it - c

Question

I'm trying to make .rodata section location stay with its associated function memory location. I'm using the GNU compiler/linker, bare metal, plain-jane c, with an STM32L4A6 micro-controller.
I have a custom board using an STM32L4A6 controller with 1Meg of Flash divided into 512 - 2K pages. Each page can be individually erased and programmed from a function running in RAM. I'd like to take advantage of this fine-grained flash organization to create an embedded firmware application that can be updated on-the-fly by modifying or adding individual functions in the code. My scheme is to dedicate a separate page of flash for each function that might ever need to be changed or created. It's extremely wasteful of flash but I'll never use more than ~10% of it so I can afford to to be wasteful. I've made some progress on this and can now make significant changes to the operation of my application by uploading very small bits of binary code. These "Patches" often do not even require a system reboot.
The problem I'm having is that when a function contains any sort of constant data, such as a literal string, it winds up in the .rodata section. I need the rodata for a given function to stay in the same area as the function that created it. Does anyone know how I might be able to force the .rodata that is created in a function to stay attached to that same function in flash? Like maybe the .rodata from that function could be positioned immediately following the function itself? Maybe I need to use -ffunction-sections or something like that? I've been through the various linker manuals but still can't figure how to do this. Below is the start of my linker script. I don't know how to include function .rodata in the individual page sections.
Example function:
#define P018 __attribute__((long_call, section(".txt018")))
P018 int Function18(int A, int B){int C = A*B; return C;}
A better example that shows my problem would be the following:
#define P152 __attribute__((long_call, section(".txt152")))
P152 void TestFunc(int A){printf("%d Squared Is: %d\r\n",A,A*A);}
In this case, the binary equivalent of "%d Squared Is: %d\r\n" can be found in .rodata with all of the other literal strings in my program. I would prefer it to be located in section .txt152 .
Linker Script snippet (Mostly generated from a simple console program.)
MEMORY
{
p000 (rx) : ORIGIN = 0x08000000, LENGTH = 0x8000
p016 (rx) : ORIGIN = 0x08008000, LENGTH = 0x800
p017 (rx) : ORIGIN = 0x08008800, LENGTH = 0x800
p018 (rx) : ORIGIN = 0x08009000, LENGTH = 0x800
.
.
.
p509 (rx) : ORIGIN = 0x080fe800, LENGTH = 0x800
p510 (rx) : ORIGIN = 0x080ff000, LENGTH = 0x800
p511 (rx) : ORIGIN = 0x080ff800, LENGTH = 0x800
ram (rwx) : ORIGIN = 0x20000000, LENGTH = 256K
ram2 (rw) : ORIGIN = 0x10000000, LENGTH = 64K
}
SECTIONS
{
.vectors :
{
KEEP(*(.isr_vector .isr_vector.*))
} > p000
.txt016 : { *(.txt016) } > p016 /* first usable 2k page following 32k p000 */
.txt017 : { *(.txt017) } > p017
.txt018 : { *(.txt018) } > p018
.
.
.
.txt509 : { *(.txt509) } > p509
.txt510 : { *(.txt510) } > p510
.txt511 : { *(.txt511) } > p511
.text :
{
*(.text .text.* .gnu.linkonce.t.*)
*(.glue_7t) *(.glue_7)
*(.rodata .rodata* .gnu.linkonce.r.*)
} > p000
.
.
.
In case anyone's interested, here's my RAM code for doing the erase/program operation
__attribute__((long_call, section(".data")))
void CopyPatch
(
unsigned short Page,
unsigned int NumberOfBytesToFlash,
unsigned char *PatchBuf
)
{
unsigned int i;
unsigned long long int *Flash;
__ASM volatile ("cpsid i" : : : "memory"); //disable interrupts
Flash = (unsigned long long int *)(FLASH_BASE + Page*2048); //set flash memory pointer to Page address
GPIOE->BSRR = GPIO_BSRR_BS_1; //make PE1(LED) high
FLASH->KEYR = 0x45670123; //unlock the flash
FLASH->KEYR = 0xCDEF89AB; //unlock the flash
while(FLASH->SR & FLASH_SR_BSY){} //wait while flash memory operation is in progress
FLASH->CR = FLASH_CR_PER | (Page << 3); //set Page erase bit and the Page to erase
FLASH->CR |= FLASH_CR_STRT; //start erase of Page
while(FLASH->SR & FLASH_SR_BSY){} //wait while Flash memory operation is in progress
FLASH->CR = FLASH_CR_PG; //set flash programming bit
for(i=0;i<(NumberOfBytesToFlash/8+1);i++)
{
Flash[i] = ((unsigned long long int *)PatchBuf)[i]; //copy RAM to FLASH, 8 bytes at a time
while(FLASH->SR & FLASH_SR_BSY){} //wait while flash memory operation is in progress
}
FLASH->CR = FLASH_CR_LOCK; //lock the flash
GPIOE->BSRR = GPIO_BSRR_BR_1; //make PE1(LED) low
__ASM volatile ("cpsie i" : : : "memory"); //enable interrupts
}

Answer 1

Okay ... Sorry for the delay, but I had to think about this a bit ...
I'm not sure you can do this [completely] with a linker script alone. It might be possible, but I think there's an easier/surer way [with a bit of extra prep]
A method I've used before is to compile with -S to get a .s file. Change/mangle that. And, then, compile the modified .s
Note that you may have some trouble with a global like:
int B;
This will go to a .comm section in the asm source. This may not be ideal.
For initialized data:
int B = 23;
You may want to add a section attribute to force it to a special section. Otherwise, it will end up in a .data section
So, I might avoid .comm and/or .bss sections in favor of always using initialized data. That's because .comm has the same issue as .rodata (i.e. it ends up as one big blob).
Anyway, below is a step by step process.
I put the section name macros in a common file (e.g.) sctname.h:
#define _SCTJOIN(_pre,_sct) _pre #_sct
#define _TXTSCT(_sct) __attribute__((section(_SCTJOIN(".txt",_sct))))
#define _DATSCT(_sct) __attribute__((section(_SCTJOIN(".dat",_sct))))
#ifdef SCTNO
#define TXTSCT _TXTSCT(SCTNO)
#define DATSCT _DATSCT(SCTNO)
#endif
Here's a slightly modified version of your .c file (e.g. module.c):
#include <stdio.h>
#ifndef SCTNO
#define SCTNO 152
#endif
#include "sctname.h"
int B DATSCT = 23;
TXTSCT void
TestFunc(int A)
{
printf("%d Squared Is: %d\r\n", A, A * A * B);
}
To create the .s file, we do:
cc -S -Wall -Werror -O2 module.c
The actual section name/number can be specified on the command line:
cc -S -Wall -Werror -O2 -DSCTNO=152 module.c
This gives us a module.s:
.file "module.c"
.text
.section .rodata.str1.1,"aMS",#progbits,1
.LC0:
.string "%d Squared Is: %d\r\n"
.section .txt152,"ax",#progbits
.p2align 4,,15
.globl TestFunc
.type TestFunc, #function
TestFunc:
.LFB11:
.cfi_startproc
movl %edi, %edx
movl %edi, %esi
xorl %eax, %eax
imull %edi, %edx
movl $.LC0, %edi
imull B(%rip), %edx
jmp printf
.cfi_endproc
.LFE11:
.size TestFunc, .-TestFunc
.globl B
.section .dat152,"aw"
.align 4
.type B, #object
.size B, 4
B:
.long 23
.ident "GCC: (GNU) 8.3.1 20190223 (Red Hat 8.3.1-2)"
.section .note.GNU-stack,"",#progbits
Now, we have to read in the .s and modify it. I've created a perl script that does this (e.g. rofix):
#!/usr/bin/perl
master(#ARGV);
exit(0);
sub master
{
my(#argv) = #_;
$root = shift(#argv);
$root =~ s/[.][^.]+$//;
$sfile = "$root.s";
$ofile = "$root.TMP";
open($xfsrc,"<$sfile") or
die("rofix: unable to open '$sfile' -- $!\n");
open($xfdst,">$ofile") or
die("rofix: unable to open '$sfile' -- $!\n");
$txtpre = "^[.]txt";
$datpre = "^[.]dat";
# find the text and data sections
seek($xfsrc,0,0);
while ($bf = <$xfsrc>) {
chomp($bf);
if ($bf =~ /^\s*[.]section\s(\S+)/) {
$sctcur = $1;
sctget($txtpre);
sctget($datpre);
}
}
# modify the data sections
seek($xfsrc,0,0);
while ($bf = <$xfsrc>) {
chomp($bf);
if ($bf =~ /^\s*[.]section\s(\S+)/) {
$sctcur = $1;
sctfix();
print($xfdst $bf,"\n");
next;
}
print($xfdst $bf,"\n");
}
close($xfsrc);
close($xfdst);
system("diff -u $sfile $ofile");
rename($ofile,$sfile) or
die("rofix: unable to rename '$ofile' to '$sfile' -- $!\n");
}
sub sctget
{
my($pre) = #_;
my($sctname,#sct);
{
last unless (defined($pre));
#sct = split(",",$sctcur);
$sctname = shift(#sct);
last unless ($sctname =~ /$pre/);
printf("sctget: FOUND %s\n",$sctname);
$sct_lookup{$pre} = $sctname;
}
}
sub sctfix
{
my($sctname,#sct);
my($sctnew);
{
last unless ($sctcur =~ /^[.]rodata/);
$sctnew = $sct_lookup{$txtpre};
last unless (defined($sctnew));
#sct = split(",",$sctcur);
$sctname = shift(#sct);
$sctname .= $sctnew;
unshift(#sct,$sctname);
$sctname = join(",",#sct);
$bf = sprintf("\t.section\t%s",$sctname);
}
}
The difference between the old and new module.s is:
sctget: FOUND .txt152
sctget: FOUND .dat152
--- module.s 2020-04-20 19:02:23.777302484 -0400
+++ module.TMP 2020-04-20 19:06:33.631926065 -0400
## -1,6 +1,6 ##
.file "module.c"
.text
- .section .rodata.str1.1,"aMS",#progbits,1
+ .section .rodata.txt152,"aMS",#progbits,1
.LC0:
.string "%d Squared Is: %d\r\n"
.section .txt152,"ax",#progbits
So, now, create the .o with:
cc -c module.s
For a makefile, it might be something like [with some wildcards]:
module.o: module.c
cc -S -Wall -Werror -O2 module.c
./rofix module.s
cc -c module.s
Now, you can add appropriate placements in your linker script for [your original section] .txt152 and the new .rodata.txt152.
And, the initialized data section .dat152
Note that the actual naming conventions are arbitrary. If you want to change them, just modify rofix [and the linker script] appropriately
Here's the readelf -a output for module.o:
Note that there's also a .rela.txt152 section!?!?
ELF Header:
Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
Class: ELF64
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: REL (Relocatable file)
Machine: Advanced Micro Devices X86-64
Version: 0x1
Entry point address: 0x0
Start of program headers: 0 (bytes into file)
Start of section headers: 808 (bytes into file)
Flags: 0x0
Size of this header: 64 (bytes)
Size of program headers: 0 (bytes)
Number of program headers: 0
Size of section headers: 64 (bytes)
Number of section headers: 15
Section header string table index: 14
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
[ 0] NULL 0000000000000000 00000000
0000000000000000 0000000000000000 0 0 0
[ 1] .text PROGBITS 0000000000000000 00000040
0000000000000000 0000000000000000 AX 0 0 1
[ 2] .data PROGBITS 0000000000000000 00000040
0000000000000000 0000000000000000 WA 0 0 1
[ 3] .bss NOBITS 0000000000000000 00000040
0000000000000000 0000000000000000 WA 0 0 1
[ 4] .rodata.txt152 PROGBITS 0000000000000000 00000040
0000000000000014 0000000000000001 AMS 0 0 1
[ 5] .txt152 PROGBITS 0000000000000000 00000060
000000000000001a 0000000000000000 AX 0 0 16
[ 6] .rela.txt152 RELA 0000000000000000 00000250
0000000000000048 0000000000000018 I 12 5 8
[ 7] .dat152 PROGBITS 0000000000000000 0000007c
0000000000000004 0000000000000000 WA 0 0 4
[ 8] .comment PROGBITS 0000000000000000 00000080
000000000000002d 0000000000000001 MS 0 0 1
[ 9] .note.GNU-stack PROGBITS 0000000000000000 000000ad
0000000000000000 0000000000000000 0 0 1
[10] .eh_frame PROGBITS 0000000000000000 000000b0
0000000000000030 0000000000000000 A 0 0 8
[11] .rela.eh_frame RELA 0000000000000000 00000298
0000000000000018 0000000000000018 I 12 10 8
[12] .symtab SYMTAB 0000000000000000 000000e0
0000000000000150 0000000000000018 13 11 8
[13] .strtab STRTAB 0000000000000000 00000230
000000000000001c 0000000000000000 0 0 1
[14] .shstrtab STRTAB 0000000000000000 000002b0
0000000000000078 0000000000000000 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
L (link order), O (extra OS processing required), G (group), T (TLS),
C (compressed), x (unknown), o (OS specific), E (exclude),
l (large), p (processor specific)
There are no section groups in this file.
There are no program headers in this file.
There is no dynamic section in this file.
Relocation section '.rela.txt152' at offset 0x250 contains 3 entries:
Offset Info Type Sym. Value Sym. Name + Addend
00000000000a 00050000000a R_X86_64_32 0000000000000000 .rodata.txt152 + 0
000000000011 000c00000002 R_X86_64_PC32 0000000000000000 B - 4
000000000016 000d00000004 R_X86_64_PLT32 0000000000000000 printf - 4
Relocation section '.rela.eh_frame' at offset 0x298 contains 1 entry:
Offset Info Type Sym. Value Sym. Name + Addend
000000000020 000600000002 R_X86_64_PC32 0000000000000000 .txt152 + 0
The decoding of unwind sections for machine type Advanced Micro Devices X86-64 is not currently supported.
Symbol table '.symtab' contains 14 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000000000 0 FILE LOCAL DEFAULT ABS module.c
2: 0000000000000000 0 SECTION LOCAL DEFAULT 1
3: 0000000000000000 0 SECTION LOCAL DEFAULT 2
4: 0000000000000000 0 SECTION LOCAL DEFAULT 3
5: 0000000000000000 0 SECTION LOCAL DEFAULT 4
6: 0000000000000000 0 SECTION LOCAL DEFAULT 5
7: 0000000000000000 0 SECTION LOCAL DEFAULT 7
8: 0000000000000000 0 SECTION LOCAL DEFAULT 9
9: 0000000000000000 0 SECTION LOCAL DEFAULT 10
10: 0000000000000000 0 SECTION LOCAL DEFAULT 8
11: 0000000000000000 26 FUNC GLOBAL DEFAULT 5 TestFunc
12: 0000000000000000 4 OBJECT GLOBAL DEFAULT 7 B
13: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND printf
No version information found in this file.