Re: [patch] remove MAX_ARG_PAGES
David Howells <dhowells <at> redhat.com>
2007-01-02 18:18:48 GMT
Ingo Molnar <mingo <at> elte.hu> wrote:
> FYI, i have forward ported your MAX_ARG_PAGES limit removal patch to
> 2.6.20-rc2 and have included it in the -rt kernel. It's working great -
> i can now finally do a "ls -t patches/*.patch" in my patch repository -
> something i havent been able to do for years 
I get a NULL kernel pointer reference with this patch with FRV in MMU mode.
It's whilst the kernel is attempting to execute init:
(gdb) bt
#0 0xc0063d54 in do_anonymous_page (mm=0xc05c1a04, vma=0xc05c884c, address=3221225457,
page_table=0xdc00bffc, pmd=0xc05c6f00, write_access=0) at pgtable.h:525
#1 0xc006468c in __handle_mm_fault (mm=0xbffffff1, vma=0xc05c884c, address=3691036668,
write_access=1) at memory.c:2404
#2 0xc0061c18 in get_user_pages (tsk=0xc034eba0, mm=0xc05c1a04, start=3221225457, len=1, write=1,
force=0, pages=0xc0353b80, vmas=0x0) at memory.c:1064
#3 0xc0084c7c in copy_strings (argc=0, argv=0xc0388c3c, bprm=0xc0388b88) at exec.c:326
#4 0xc0084dac in copy_strings_kernel (argc=1, argv=0xc05c884c, bprm=0xbffffff1) at exec.c:365
#5 0xc0085f5c in do_execve (filename=0xc0387000 "/sbin/init", argv=0xc026e8c8, envp=0xc026e950,
regs=0xc0353bf8) at exec.c:1158
#6 0xc000f55c in sys_execve (name=0x1 <Address 0x1 out of bounds>, argv=0xc026e8c8, envp=0xc026e950)
at process.c:263
#7 0xc000e1e0 in __syscall_call () at arch/frv/kernel/entry.S:878
#8 0xc000e1e0 in __syscall_call () at arch/frv/kernel/entry.S:878
The problem appears to be that current->mm is NULL. It's a bit difficult to
say for sure because gdb or gcc determined that the error is in pgtable.h, but
doesn't seem to have determined which one.
However, looking at the code, I think it must be this:
static int do_anonymous_page(struct mm_struct *mm, ...)
{
...
update_mmu_cache(vma, address, entry);
...
}
But that uses current->mm, which is NULL, via this function:
[include/asm-frv/pgtable.h]
/* * preload information about a newly instantiated PTE into the SCR0/SCR1 PGE
cache */
static inline void update_mmu_cache(struct vm_area_struct *vma,
unsigned long address, pte_t pte)
{
unsigned long ampr;
pgd_t *pge = pgd_offset(current->mm, address);
pud_t *pue = pud_offset(pge, address);
pmd_t *pme = pmd_offset(pue, address);
ampr = pme->ste[0] & 0xffffff00;
ampr |= xAMPRx_L | xAMPRx_SS_16Kb | xAMPRx_S | xAMPRx_C | xAMPRx_V;
asm volatile("movgs %0,scr0\n"
"movgs %0,scr1\n"
"movgs %1,dampr4\n"
"movgs %1,dampr5\n"
:
: "r"(address), "r"(ampr)
);
}
This really oughtn't to be called in this situation, I suspect. I could just
skip the operation if current->mm is NULL, but I'm not sure that that's the
right thing to do. Maybe I should be using current->active_mm instead.
Everything works fine without the patch.
David
Patch is attached below, and the problem was not in the kernel, but in glibc.
The first hunk in the patch below ("Save the PIC register.") is the fix for the arguments check which I posted
before (
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