On Tue, 13 Apr 2010, Jörgen Pihlflyckt wrote:
> I noticed that the devmem applet maps two consecutive pages of memory in
> order to allow access to a (non-aligned) object that spans two pages.
> This is a problem if we are working with the last page of memory because
> the next page would wrap around to the first page of memory.
>
> This patch makes devmem map only a single page if it is at the end of
> the memory address space.
Forgive me (and this patch isn't tested), but if you're going to do this,
let's do it right:
diff --git a/miscutils/devmem.c b/miscutils/devmem.c
index e13dedc..225a9ed 100644
--- a/miscutils/devmem.c
+++ b/miscutils/devmem.c
@@ -14,6 +14,8 @@ int devmem_main(int argc UNUSED_PARAM, char **argv)
uint64_t writeval = writeval; /* for compiler */
off_t target;
unsigned page_size = getpagesize();
+ off_t page_mask = ~(off_t)(page_size - 1);
+ size_t map_length;
int fd;
int width = 8 * sizeof(int);
@@ -57,18 +59,19 @@ int devmem_main(int argc UNUSED_PARAM, char **argv)
bb_show_usage(); /* bb_strtouXX failed */
fd = xopen("/dev/mem", argv[3] ? (O_RDWR | O_SYNC) : (O_RDONLY |
O_SYNC));
+ map_length = ((target + width - 1) & page_mask) - (target & page_mask)
+ page_size;
map_base = mmap(NULL,
- page_size * 2 /* in case value spans page */,
+ map_length,
argv[3] ? (PROT_READ | PROT_WRITE) : PROT_READ,
MAP_SHARED,
fd,
- target & ~(off_t)(page_size - 1));
+ target & page_mask);
if (map_base == MAP_FAILED)
bb_perror_msg_and_die("mmap");
// printf("Memory mapped at address %p.\n", map_base);
- virt_addr = (char*)map_base + (target & (page_size - 1));
+ virt_addr = (char*)map_base + (target & page_mask);
if (!argv[3]) {
switch (width) {
@@ -119,7 +123,7 @@ int devmem_main(int argc UNUSED_PARAM, char **argv)
}
if (ENABLE_FEATURE_CLEAN_UP) {
- if (munmap(map_base, page_size * 2) == -1)
+ if (munmap(map_base, map_length) == -1)
bb_perror_msg_and_die("munmap");
close(fd);
}
Two points: you might as well compute the page span properly, if you're
going to touch this code, and of course you do need to unmap what you've
just mapped!
*Think* I've got the calculation right, but as I said, completely
untested, sorry._______________________________________________
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