Messages in this thread | | | Date | Mon, 29 Jun 2009 13:55:13 -0700 | From | Randy Dunlap <> | Subject | Re: [RESEND Patch 1/2] Doc: update Documentation/exception.txt |
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Amerigo Wang wrote: > Update Documentation/exception.txt. > Remove trailing whitespaces in it. > > Signed-off-by: WANG Cong <amwang@redhat.com> > Cc: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Ingo, do you want to merge these or should I do it?
Thanks.
> --- > Index: linux-2.6/Documentation/exception.txt > =================================================================== > --- linux-2.6.orig/Documentation/exception.txt > +++ linux-2.6/Documentation/exception.txt > @@ -1,123 +1,123 @@ > - Kernel level exception handling in Linux 2.1.8 > + Kernel level exception handling in Linux > Commentary by Joerg Pommnitz <joerg@raleigh.ibm.com> > > -When a process runs in kernel mode, it often has to access user > -mode memory whose address has been passed by an untrusted program. > +When a process runs in kernel mode, it often has to access user > +mode memory whose address has been passed by an untrusted program. > To protect itself the kernel has to verify this address. > > -In older versions of Linux this was done with the > -int verify_area(int type, const void * addr, unsigned long size) > +In older versions of Linux this was done with the > +int verify_area(int type, const void * addr, unsigned long size) > function (which has since been replaced by access_ok()). > > -This function verified that the memory area starting at address > +This function verified that the memory area starting at address > 'addr' and of size 'size' was accessible for the operation specified > -in type (read or write). To do this, verify_read had to look up the > -virtual memory area (vma) that contained the address addr. In the > -normal case (correctly working program), this test was successful. > +in type (read or write). To do this, verify_read had to look up the > +virtual memory area (vma) that contained the address addr. In the > +normal case (correctly working program), this test was successful. > It only failed for a few buggy programs. In some kernel profiling > tests, this normally unneeded verification used up a considerable > amount of time. > > -To overcome this situation, Linus decided to let the virtual memory > +To overcome this situation, Linus decided to let the virtual memory > hardware present in every Linux-capable CPU handle this test. > > How does this work? > > -Whenever the kernel tries to access an address that is currently not > -accessible, the CPU generates a page fault exception and calls the > -page fault handler > +Whenever the kernel tries to access an address that is currently not > +accessible, the CPU generates a page fault exception and calls the > +page fault handler > > void do_page_fault(struct pt_regs *regs, unsigned long error_code) > > -in arch/i386/mm/fault.c. The parameters on the stack are set up by > -the low level assembly glue in arch/i386/kernel/entry.S. The parameter > -regs is a pointer to the saved registers on the stack, error_code > +in arch/x86/mm/fault.c. The parameters on the stack are set up by > +the low level assembly glue in arch/x86/kernel/entry_32.S. The parameter > +regs is a pointer to the saved registers on the stack, error_code > contains a reason code for the exception. > > -do_page_fault first obtains the unaccessible address from the CPU > -control register CR2. If the address is within the virtual address > -space of the process, the fault probably occurred, because the page > -was not swapped in, write protected or something similar. However, > -we are interested in the other case: the address is not valid, there > -is no vma that contains this address. In this case, the kernel jumps > -to the bad_area label. > - > -There it uses the address of the instruction that caused the exception > -(i.e. regs->eip) to find an address where the execution can continue > -(fixup). If this search is successful, the fault handler modifies the > -return address (again regs->eip) and returns. The execution will > +do_page_fault first obtains the unaccessible address from the CPU > +control register CR2. If the address is within the virtual address > +space of the process, the fault probably occurred, because the page > +was not swapped in, write protected or something similar. However, > +we are interested in the other case: the address is not valid, there > +is no vma that contains this address. In this case, the kernel jumps > +to the bad_area label. > + > +There it uses the address of the instruction that caused the exception > +(i.e. regs->eip) to find an address where the execution can continue > +(fixup). If this search is successful, the fault handler modifies the > +return address (again regs->eip) and returns. The execution will > continue at the address in fixup. > > Where does fixup point to? > > -Since we jump to the contents of fixup, fixup obviously points > -to executable code. This code is hidden inside the user access macros. > -I have picked the get_user macro defined in include/asm/uaccess.h as an > -example. The definition is somewhat hard to follow, so let's peek at > +Since we jump to the contents of fixup, fixup obviously points > +to executable code. This code is hidden inside the user access macros. > +I have picked the get_user macro defined in arch/x86/include/asm/uaccess.h > +as an example. The definition is somewhat hard to follow, so let's peek at > the code generated by the preprocessor and the compiler. I selected > -the get_user call in drivers/char/console.c for a detailed examination. > +the get_user call in drivers/char/sysrq.c for a detailed examination. > > -The original code in console.c line 1405: > +The original code in sysrq.c line 587: > get_user(c, buf); > > The preprocessor output (edited to become somewhat readable): > > ( > - { > - long __gu_err = - 14 , __gu_val = 0; > - const __typeof__(*( ( buf ) )) *__gu_addr = ((buf)); > - if (((((0 + current_set[0])->tss.segment) == 0x18 ) || > - (((sizeof(*(buf))) <= 0xC0000000UL) && > - ((unsigned long)(__gu_addr ) <= 0xC0000000UL - (sizeof(*(buf))))))) > + { > + long __gu_err = - 14 , __gu_val = 0; > + const __typeof__(*( ( buf ) )) *__gu_addr = ((buf)); > + if (((((0 + current_set[0])->tss.segment) == 0x18 ) || > + (((sizeof(*(buf))) <= 0xC0000000UL) && > + ((unsigned long)(__gu_addr ) <= 0xC0000000UL - (sizeof(*(buf))))))) > do { > - __gu_err = 0; > - switch ((sizeof(*(buf)))) { > - case 1: > - __asm__ __volatile__( > - "1: mov" "b" " %2,%" "b" "1\n" > - "2:\n" > - ".section .fixup,\"ax\"\n" > - "3: movl %3,%0\n" > - " xor" "b" " %" "b" "1,%" "b" "1\n" > - " jmp 2b\n" > - ".section __ex_table,\"a\"\n" > - " .align 4\n" > - " .long 1b,3b\n" > + __gu_err = 0; > + switch ((sizeof(*(buf)))) { > + case 1: > + __asm__ __volatile__( > + "1: mov" "b" " %2,%" "b" "1\n" > + "2:\n" > + ".section .fixup,\"ax\"\n" > + "3: movl %3,%0\n" > + " xor" "b" " %" "b" "1,%" "b" "1\n" > + " jmp 2b\n" > + ".section __ex_table,\"a\"\n" > + " .align 4\n" > + " .long 1b,3b\n" > ".text" : "=r"(__gu_err), "=q" (__gu_val): "m"((*(struct __large_struct *) > - ( __gu_addr )) ), "i"(- 14 ), "0"( __gu_err )) ; > - break; > - case 2: > + ( __gu_addr )) ), "i"(- 14 ), "0"( __gu_err )) ; > + break; > + case 2: > __asm__ __volatile__( > - "1: mov" "w" " %2,%" "w" "1\n" > - "2:\n" > - ".section .fixup,\"ax\"\n" > - "3: movl %3,%0\n" > - " xor" "w" " %" "w" "1,%" "w" "1\n" > - " jmp 2b\n" > - ".section __ex_table,\"a\"\n" > - " .align 4\n" > - " .long 1b,3b\n" > + "1: mov" "w" " %2,%" "w" "1\n" > + "2:\n" > + ".section .fixup,\"ax\"\n" > + "3: movl %3,%0\n" > + " xor" "w" " %" "w" "1,%" "w" "1\n" > + " jmp 2b\n" > + ".section __ex_table,\"a\"\n" > + " .align 4\n" > + " .long 1b,3b\n" > ".text" : "=r"(__gu_err), "=r" (__gu_val) : "m"((*(struct __large_struct *) > - ( __gu_addr )) ), "i"(- 14 ), "0"( __gu_err )); > - break; > - case 4: > - __asm__ __volatile__( > - "1: mov" "l" " %2,%" "" "1\n" > - "2:\n" > - ".section .fixup,\"ax\"\n" > - "3: movl %3,%0\n" > - " xor" "l" " %" "" "1,%" "" "1\n" > - " jmp 2b\n" > - ".section __ex_table,\"a\"\n" > - " .align 4\n" " .long 1b,3b\n" > + ( __gu_addr )) ), "i"(- 14 ), "0"( __gu_err )); > + break; > + case 4: > + __asm__ __volatile__( > + "1: mov" "l" " %2,%" "" "1\n" > + "2:\n" > + ".section .fixup,\"ax\"\n" > + "3: movl %3,%0\n" > + " xor" "l" " %" "" "1,%" "" "1\n" > + " jmp 2b\n" > + ".section __ex_table,\"a\"\n" > + " .align 4\n" " .long 1b,3b\n" > ".text" : "=r"(__gu_err), "=r" (__gu_val) : "m"((*(struct __large_struct *) > - ( __gu_addr )) ), "i"(- 14 ), "0"(__gu_err)); > - break; > - default: > - (__gu_val) = __get_user_bad(); > - } > - } while (0) ; > - ((c)) = (__typeof__(*((buf))))__gu_val; > + ( __gu_addr )) ), "i"(- 14 ), "0"(__gu_err)); > + break; > + default: > + (__gu_val) = __get_user_bad(); > + } > + } while (0) ; > + ((c)) = (__typeof__(*((buf))))__gu_val; > __gu_err; > } > ); > @@ -127,12 +127,12 @@ see what code gcc generates: > > > xorl %edx,%edx > > movl current_set,%eax > - > cmpl $24,788(%eax) > - > je .L1424 > + > cmpl $24,788(%eax) > + > je .L1424 > > cmpl $-1073741825,64(%esp) > - > ja .L1423 > + > ja .L1423 > > .L1424: > - > movl %edx,%eax > + > movl %edx,%eax > > movl 64(%esp),%ebx > > #APP > > 1: movb (%ebx),%dl /* this is the actual user access */ > @@ -149,17 +149,17 @@ see what code gcc generates: > > .L1423: > > movzbl %dl,%esi > > -The optimizer does a good job and gives us something we can actually > -understand. Can we? The actual user access is quite obvious. Thanks > -to the unified address space we can just access the address in user > +The optimizer does a good job and gives us something we can actually > +understand. Can we? The actual user access is quite obvious. Thanks > +to the unified address space we can just access the address in user > memory. But what does the .section stuff do????? > > To understand this we have to look at the final kernel: > > > objdump --section-headers vmlinux > - > > + > > > vmlinux: file format elf32-i386 > - > > + > > > Sections: > > Idx Name Size VMA LMA File off Algn > > 0 .text 00098f40 c0100000 c0100000 00001000 2**4 > @@ -198,18 +198,18 @@ final kernel executable: > > The whole user memory access is reduced to 10 x86 machine instructions. > The instructions bracketed in the .section directives are no longer > -in the normal execution path. They are located in a different section > +in the normal execution path. They are located in a different section > of the executable file: > > > objdump --disassemble --section=.fixup vmlinux > - > > + > > > c0199ff5 <.fixup+10b5> movl $0xfffffff2,%eax > > c0199ffa <.fixup+10ba> xorb %dl,%dl > > c0199ffc <.fixup+10bc> jmp c017e7a7 <do_con_write+e3> > > And finally: > > objdump --full-contents --section=__ex_table vmlinux > - > > + > > > c01aa7c4 93c017c0 e09f19c0 97c017c0 99c017c0 ................ > > c01aa7d4 f6c217c0 e99f19c0 a5e717c0 f59f19c0 ................ > > c01aa7e4 080a18c0 01a019c0 0a0a18c0 04a019c0 ................ > @@ -235,8 +235,8 @@ sections in the ELF object file. So the > ended up in the .fixup section of the object file and the addresses > .long 1b,3b > ended up in the __ex_table section of the object file. 1b and 3b > -are local labels. The local label 1b (1b stands for next label 1 > -backward) is the address of the instruction that might fault, i.e. > +are local labels. The local label 1b (1b stands for next label 1 > +backward) is the address of the instruction that might fault, i.e. > in our case the address of the label 1 is c017e7a5: > the original assembly code: > 1: movb (%ebx),%dl > and linked in vmlinux : > c017e7a5 <do_con_write+e1> movb (%ebx),%dl > @@ -254,7 +254,7 @@ The assembly code > becomes the value pair > > c01aa7d4 c017c2f6 c0199fe9 c017e7a5 c0199ff5 ................ > ^this is ^this is > - 1b 3b > + 1b 3b > c017e7a5,c0199ff5 in the exception table of the kernel. > > So, what actually happens if a fault from kernel mode with no suitable > @@ -266,9 +266,9 @@ vma occurs? > 3.) CPU calls do_page_fault > 4.) do page fault calls search_exception_table (regs->eip == c017e7a5); > 5.) search_exception_table looks up the address c017e7a5 in the > - exception table (i.e. the contents of the ELF section __ex_table) > + exception table (i.e. the contents of the ELF section __ex_table) > and returns the address of the associated fault handle code c0199ff5. > -6.) do_page_fault modifies its own return address to point to the fault > +6.) do_page_fault modifies its own return address to point to the fault > handle code and returns. > 7.) execution continues in the fault handling code. > 8.) 8a) EAX becomes -EFAULT (== -14)
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