Hello, fdlength in lib.c does not seem to work properly, I noticed this when toybox's insmod sometimes failed mysteriously. I have attached a patch that should fix the issue, along with a test case that evidences the issue (use -std=c99).
Regards, Achille
patch-fix-fdlength.diff
Description: Binary data
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <errno.h>
#include <fcntl.h>
#include <assert.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
// Return how long the file at fd is, if there's any way to determine it.
off_t fdlength_old(int fd)
{
off_t bottom = 0, top = 0, pos, old;
int size;
// If not, do a binary search for the last location we can read.
old = lseek(fd, 0, SEEK_CUR);
do {
char temp;
pos = bottom + (top - bottom) / 2;
// If we can read from the current location, it's bigger.
if (lseek(fd, pos, 0)>=0 && read(fd, &temp, 1)==1) {
if (bottom == top) bottom = top = (top+1) * 2;
else bottom = pos;
// If we can't, it's smaller.
} else {
if (bottom == top) {
if (!top) return 0;
bottom = top/2;
} else top = pos;
}
} while (bottom + 1 != top);
lseek(fd, old, SEEK_SET);
return pos + 1;
}
static bool testread(int fd, int pos)
{
char temp;
return lseek(fd, pos, SEEK_SET) >= 0 &&
read(fd, &temp, 1) == 1;
}
int fdlength_new(int fd)
{
int old = lseek(fd, 0, SEEK_CUR);
#ifdef CHEAT
int n = lseek(fd, 0, SEEK_END);
if (n >= 0) {
lseek(fd, old, SEEK_SET);
return n;
}
#endif
int bottom = 0, top = 0;
// ensure top is not readable
while (testread(fd, top)) {
top = (top + 1) * 2; // XXX: might overflow
}
if (top == 0)
return 0;
for (;;) {
int pos = bottom + (top - bottom) / 2;
if (testread(fd, pos)) {
if (pos + 1 == top) {
lseek(fd, old, SEEK_SET);
return top;
}
bottom = pos;
} else {
top = pos;
}
}
}
static const char zero_path[] = "/tmp/zero";
static int zero_create(int size)
{
int fd = open(zero_path, O_WRONLY | O_CREAT | O_TRUNC, S_IRUSR | S_IWUSR);
if (fd < 0)
return fd;
char block[4096];
memset(block, 0, sizeof(block));
while (size > 0) {
int n = size <= (int)sizeof(block) ? size : (int)sizeof(block);
write(fd, block, n);
size -= n;
}
close(fd);
return open(zero_path, O_RDONLY);
}
static void zero_delete()
{
unlink(zero_path);
}
int main()
{
for (int i = 0; i < 1024; ++i) {
int fd = zero_create(i);
if (fd < 0)
return 1;
int n_old = fdlength_old(fd);
int n_new = fdlength_new(fd);
assert(n_new == i);
if (n_old != i)
printf("length = %d, n_old = %d, n_new = %d\n", i, n_old, n_new);
close(fd);
zero_delete();
}
return 0;
}
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