Page fault with newlib functions
-
19-03-2021 - |
Question
I've been porting newlib to my very small kernel, and I'm stumped: whenever I include a function that references a system call, my program will page fault on execution. If I call a function that does not reference a system call, like rand()
, nothing will go wrong.
Note: By include, I mean as long as the function, e.g. printf()
or fopen()
, is somewhere inside the program, even if it isn't called through main()
.
I've had this problem for quite some time now, and have no idea what could be causing this:
- I've rebuilt newlib numerous times
- Modified my ELF loader to load the code from the section headers instead of program headers
- Attempted to build newlib/libgloss separately (which failed)
- Linked the libraries (libc, libnosys) through the ld script using
GROUP
, gcc and ld
I'm not quite sure what other information I should include with this, but I'd be happy to include what I can.
Edit: To verify, the page faults occurring are not at the addresses of the failing functions; they are elsewhere in the program. For example, when I call fopen()
, located at 0x08048170, I will page fault at 0xA00A316C.
Edit 2: Relevant code for loading ELF:
int krun(u8int *name) {
int fd = kopen(name);
Elf32_Ehdr *ehdr = kmalloc(sizeof(Elf32_Ehdr*));
read(fd, ehdr, sizeof(Elf32_Ehdr));
if (ehdr->e_ident[0] != 0x7F || ehdr->e_ident[1] != 'E' || ehdr->e_ident[2] != 'L' || ehdr->e_ident[3] != 'F') {
kfree(ehdr);
return -1;
}
int pheaders = ehdr->e_phnum;
int phoff = ehdr->e_phoff;
int phsize = ehdr->e_phentsize;
int sheaders = ehdr->e_shnum;
int shoff = ehdr->e_shoff;
int shsize = ehdr->e_shentsize;
for (int i = 0; i < pheaders; i++) {
lseek(fd, phoff + phsize * i, SEEK_SET);
Elf32_Phdr *phdr = kmalloc(sizeof(Elf32_Phdr*));
read(fd, phdr, sizeof(Elf32_Phdr));
u32int page = PMMAllocPage();
int flags = 0;
if (phdr->p_flags & PF_R) flags |= PAGE_PRESENT;
if (phdr->p_flags & PF_W) flags |= PAGE_WRITE;
int pages = (phdr->p_memsz / 0x1000) + 1;
while (pages >= 0) {
u32int mapaddr = (phdr->p_vaddr + (pages * 0x1000)) & 0xFFFFF000;
map(mapaddr, page, flags | PAGE_USER);
pages--;
}
lseek(fd, phdr->p_offset, SEEK_SET);
read(fd, (void *)phdr->p_vaddr, phdr->p_filesz);
kfree(phdr);
}
// Removed: code block that zeroes .bss: it's already zeroed whenever I check it anyways
// Removed: code block that creates thread and adds it to scheduler
kfree(ehdr);
return 0;
}
Edit 3: I've noticed that if I call a system call, such as write()
, and then call printf()
two or more times, I will get an unknown opcode interrupt. Odd.
Solution
Whoops! Figured it out: when I map the virtual address, I should allocate a new page each time, like so:
map(mapaddr, PMMAllocPage(), flags | PAGE_USER);
Now it works fine.
For those curious as to why it didn't work: when I wasn't including printf()
, the size of the program was under 0x1000 bytes, so mapping with only one page was okay. When I include printf()
or fopen()
, the size of the program was much bigger so that's what caused the issue.