On 01/07/2007 11:20 AM, Jeremy Fitzhardinge wrote:
Rene Herman wrote:
How is it for efficiency? I thought it was for correctness.
romsignature is using probe_kernel_adress() while all other accesses
to the ROMs there aren't.
If nothing else, anyone reading that code is likely to ask himself the
very same question -- why the one, and not the others.
Well, I was wondering about all the uses of __get_user; why not
probe_kernel_address() everywhere?
It's just a manual version of probe_kernel_adress():
#define probe_kernel_address(addr, retval) \
({ \
long ret; \
mm_segment_t old_fs = get_fs(); \
\
set_fs(KERNEL_DS); \
pagefault_disable(); \
ret = __get_user(retval, [ ... ]); \
pagefault_enable(); \
set_fs(old_fs); \
ret; \
})
Doing the set_fs() and pagefault_{disable,enable} calls for every single
byte during the checksum seems rather silly. The patch as posted has the
set_fs() and pagefault_ calls only once in probe_roms() (as said when
posted, I'm not sure the pagefault calls are still useful now that it's
no longer a generic function/macro, but used directly at probe_roms() time).
I think its reasonable to assume that if the signature is mapped and
correct, then everything else is mapped. That's certainly the case
for Xen, which is why I added it. If you think this is unclear, then
I think a comment to explain this rather than code changes is the
appropriate fix.
I disagree I'm afraid. Given what __get_user compiles to (nothing more
than a .fixup entry, basically) they're largely "free" and it makes the
code completely obvious: "If you're touching this, do so via __get_user
and not directly" and frees it from any assumptions, however reasonable
or unreasonable.
Would you _mind_ if I submit it? If not, if you could comment on whether
or not these pagefault calls are still useful, that would be great. The
comment at probe_kernel_address() says:
* We ensure that the __get_user() is executed in atomic context so that
* do_page_fault() doesn't attempt to take mmap_sem. This makes
* probe_kernel_address() suitable for use within regions where the
* caller already holds mmap_sem, or other locks which nest inside
* mmap_sem
This sounds like it might be nonsensical at probe_roms() time, but I'm
not familiar with these virtualized environments -- I do not know which
assumptions break.
Patch attached again for reference...
Rene.
commit f153a588097c08cefdb799f22123192a9975d273
Author: Rene Herman <[EMAIL PROTECTED]>
Date: Sat Jan 6 04:09:32 2007 +0100
Use __get_user() for ISA ROM accesses.
In virtualized environments, the ISA ROMs may not be mapped so be careful
about touching them.
Signed-off-by: Rene Herman <[EMAIL PROTECTED]>
diff --git a/arch/i386/kernel/e820.c b/arch/i386/kernel/e820.c
index f391abc..8b54f65 100644
--- a/arch/i386/kernel/e820.c
+++ b/arch/i386/kernel/e820.c
@@ -156,29 +156,34 @@ static struct resource standard_io_resou
.flags = IORESOURCE_BUSY | IORESOURCE_IO
} };
-static int romsignature(const unsigned char *x)
+#define ROM_SIG 0xaa55
+
+static int __init romsignature(const unsigned char *rom)
{
unsigned short sig;
- int ret = 0;
- if (probe_kernel_address((const unsigned short *)x, sig) == 0)
- ret = (sig == 0xaa55);
- return ret;
+
+ return !__get_user(sig, (const unsigned short *)rom) && sig == ROM_SIG;
}
-static int __init romchecksum(unsigned char *rom, unsigned long length)
+static int __init romchecksum(const unsigned char *rom, unsigned long length)
{
- unsigned char *p, sum = 0;
+ unsigned char sum, c;
- for (p = rom; p < rom + length; p++)
- sum += *p;
- return sum == 0;
+ for (sum = 0; length && !__get_user(c, rom); rom++, length--)
+ sum += c;
+ return !length && !sum;
}
static void __init probe_roms(void)
{
+ const unsigned char *rom;
unsigned long start, length, upper;
- unsigned char *rom;
- int i;
+ unsigned char c;
+ int i;
+ mm_segment_t old_fs = get_fs();
+
+ set_fs(KERNEL_DS);
+ pagefault_disable();
/* video rom */
upper = adapter_rom_resources[0].start;
@@ -189,8 +194,11 @@ static void __init probe_roms(void)
video_rom_resource.start = start;
+ if (__get_user(c, rom + 2))
+ continue;
+
/* 0 < length <= 0x7f * 512, historically */
- length = rom[2] * 512;
+ length = c * 512;
/* if checksum okay, trust length byte */
if (length && romchecksum(rom, length))
@@ -224,8 +232,11 @@ static void __init probe_roms(void)
if (!romsignature(rom))
continue;
+ if (__get_user(c, rom + 2))
+ continue;
+
/* 0 < length <= 0x7f * 512, historically */
- length = rom[2] * 512;
+ length = c * 512;
/* but accept any length that fits if checksum okay */
if (!length || start + length > upper || !romchecksum(rom,
length))
@@ -237,6 +248,9 @@ static void __init probe_roms(void)
start = adapter_rom_resources[i++].end & ~2047UL;
}
+
+ pagefault_enable();
+ set_fs(old_fs);
}
/*
