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syscall: implement getdents64
V2: The only difference is removed delete_dir() function was accidentally left from previous attempts to implement this syscall. It looks like the golang apps that need to iterate over entries in a directory use a system call getdents64 which is documented in https://man7.org/linux/man-pages/man2/getdents.2.html. Normally this functionality is provided by the libc functions like opendir(), readdir(), etc which actually do delegate to getdents64. Go is known of bypassing libc in such cases. So this patch implements the syscall getdents64 by adding a utility function to VFS main.cc that is then called by syscall in linux.cc. For details of how this function works please look at the comments. This patch also adds a unit test to verify this syscall works. Refs #1188 Signed-off-by: Waldemar Kozaczuk <[email protected]>
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‎fs/vfs/main.cc‎

Lines changed: 65 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -790,6 +790,71 @@ int readdir64_r(DIR *dir, struct dirent64 *entry,
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extern "C" OSV_LIBC_API
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struct dirent *readdir64(DIR *dir) __attribute__((alias("readdir")));
792792

793+
struct linux_dirent64 {
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u64 d_ino;
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s64 d_off;
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unsigned short d_reclen;
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unsigned char d_type;
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char d_name[];
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};
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#undef getdents64
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extern "C"
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ssize_t sys_getdents64(int fd, void *dirp, size_t count)
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{
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auto *dir = fdopendir(fd);
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if (dir) {
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// We have verified that fd points to a valid directory
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// but we do NOT need the DIR handle so just delete it
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delete dir;
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struct file *fp;
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int error = fget(fd, &fp);
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if (error) {
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errno = error;
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return -1;
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}
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size_t bytes_read = 0;
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off_t last_off = -1;
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errno = 0;
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// Iterate over as many entries as there is space in the buffer
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// by directly calling sys_readdir()
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struct dirent entry;
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while ((error = sys_readdir(fp, &entry)) == 0) {
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auto rec_len = offsetof(linux_dirent64, d_name) + strlen(entry.d_name) + 1;
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if (rec_len <= count) {
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auto *ldirent = static_cast<linux_dirent64*>(dirp + bytes_read);
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ldirent->d_ino = entry.d_ino;
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ldirent->d_off = entry.d_off;
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ldirent->d_type = entry.d_type;
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strcpy(ldirent->d_name, entry.d_name);
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ldirent->d_reclen = rec_len;
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count -= rec_len;
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bytes_read += rec_len;
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last_off = entry.d_off;
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} else {
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if (last_off >= 0)
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sys_seekdir(fp, last_off);
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break;
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}
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}
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fdrop(fp);
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if (error && error != ENOENT) {
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errno = error;
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return -1;
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} else {
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errno = 0;
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return bytes_read;
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}
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} else {
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return -1;
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}
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}
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793858
OSV_LIBC_API
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void rewinddir(DIR *dirp)
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{

‎linux.cc‎

Lines changed: 4 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -424,6 +424,9 @@ static int tgkill(int tgid, int tid, int sig)
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return -1;
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}
426426

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#define __NR_sys_getdents64 __NR_getdents64
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extern "C" ssize_t sys_getdents64(int fd, void *dirp, size_t count);
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427430
OSV_LIBC_API long syscall(long number, ...)
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{
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// Save FPU state and restore it at the end of this function
@@ -512,6 +515,7 @@ OSV_LIBC_API long syscall(long number, ...)
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SYSCALL2(statfs, const char *, struct statfs *);
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SYSCALL3(unlinkat, int, const char *, int);
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SYSCALL3(symlinkat, const char *, int, const char *);
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SYSCALL3(sys_getdents64, int, void *, size_t);
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}
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debug_always("syscall(): unimplemented system call %d\n", number);

‎modules/tests/Makefile‎

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -133,7 +133,7 @@ tests := tst-pthread.so misc-ramdisk.so tst-vblk.so tst-bsd-evh.so \
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tst-getopt.so tst-getopt-pie.so tst-non-pie.so tst-semaphore.so \
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tst-elf-init.so tst-realloc.so tst-setjmp.so \
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libtls.so libtls_gold.so tst-tls.so tst-tls-gold.so tst-tls-pie.so \
136-
tst-sigaction.so tst-syscall.so tst-ifaddrs.so
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tst-sigaction.so tst-syscall.so tst-ifaddrs.so tst-getdents.so
137137
# libstatic-thread-variable.so tst-static-thread-variable.so \
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139139
#TODO For now let us disable these tests for aarch64 until

‎tests/tst-getdents.cc‎

Lines changed: 111 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,111 @@
1+
#include <dirent.h> /* Defines DT_* constants */
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#include <fcntl.h>
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#include <stdint.h>
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#include <stdio.h>
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#include <unistd.h>
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#include <stdlib.h>
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#include <sys/stat.h>
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#include <sys/syscall.h>
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#include <cassert>
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#include <memory>
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#include <string>
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#include <vector>
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#include <algorithm>
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16+
#define handle_error(msg) \
17+
do { perror(msg); exit(EXIT_FAILURE); } while (0)
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struct test_dirent64 {
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unsigned long d_ino;
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off_t d_off;
22+
unsigned char d_type;
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std::string d_name;
24+
25+
bool operator ==(const test_dirent64 &b) const {
26+
return d_ino == b.d_ino &&
27+
d_off == b.d_off &&
28+
d_type == b.d_type &&
29+
d_name == b.d_name;
30+
}
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};
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// This code is loosely based on the example found under https://man7.org/linux/man-pages/man2/getdents.2.html
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void test_getdents64(const char *dir_path, size_t buf_size, std::vector<test_dirent64> &dirents) {
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struct linux_dirent64 {
36+
unsigned long d_ino;
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off_t d_off;
38+
unsigned short d_reclen;
39+
unsigned char d_type;
40+
char d_name[];
41+
};
42+
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int fd = open(dir_path, O_RDONLY | O_DIRECTORY);
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if (fd == -1)
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handle_error("open");
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std::unique_ptr<char []> buf_ptr(new char[buf_size]);
48+
char *buf = buf_ptr.get();
49+
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for (;;) {
51+
long nread = syscall(SYS_getdents64, fd, buf, buf_size);
52+
if (nread == -1)
53+
handle_error("getdents64");
54+
55+
if (nread == 0)
56+
break;
57+
58+
printf("--------------- nread=%ld ---------------\n", nread);
59+
printf("inode# file type d_reclen d_off d_name\n");
60+
for (long bpos = 0; bpos < nread;) {
61+
auto *d = (struct linux_dirent64 *) (buf + bpos);
62+
printf("%8ld ", d->d_ino);
63+
64+
char d_type = d->d_type;
65+
printf("%-10s ", (d_type == DT_REG) ? "regular" :
66+
(d_type == DT_DIR) ? "directory" :
67+
(d_type == DT_FIFO) ? "FIFO" :
68+
(d_type == DT_SOCK) ? "socket" :
69+
(d_type == DT_LNK) ? "symlink" :
70+
(d_type == DT_BLK) ? "block dev" :
71+
(d_type == DT_CHR) ? "char dev" : "???");
72+
73+
printf("%4d %10jd %s\n", d->d_reclen,
74+
(intmax_t) d->d_off, d->d_name);
75+
bpos += d->d_reclen;
76+
77+
test_dirent64 dirent;
78+
dirent.d_ino = d->d_ino;
79+
dirent.d_off = d->d_off;
80+
dirent.d_type = d_type;
81+
dirent.d_name = d->d_name;
82+
dirents.push_back(dirent);
83+
}
84+
}
85+
86+
close(fd);
87+
}
88+
89+
#define LARGE_BUF_SIZE 1024
90+
#define SMALL_BUF_SIZE 128
91+
92+
int main()
93+
{
94+
// Verify that getdents64 works correctly against /proc directory and yields
95+
// correct results
96+
std::vector<test_dirent64> dirents_1;
97+
test_getdents64("/proc", LARGE_BUF_SIZE, dirents_1);
98+
99+
assert(std::count_if(dirents_1.begin(), dirents_1.end(), [](test_dirent64 d) { return d.d_type == DT_REG; }) >= 3);
100+
assert(std::count_if(dirents_1.begin(), dirents_1.end(), [](test_dirent64 d) { return d.d_type == DT_DIR; }) >= 5);
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102+
assert(std::find_if(dirents_1.begin(), dirents_1.end(), [](test_dirent64 d) { return d.d_name == ".."; }) != dirents_1.end());
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assert(std::find_if(dirents_1.begin(), dirents_1.end(), [](test_dirent64 d) { return d.d_name == "cpuinfo"; }) != dirents_1.end());
104+
assert(std::find_if(dirents_1.begin(), dirents_1.end(), [](test_dirent64 d) { return d.d_name == "sys"; }) != dirents_1.end());
105+
106+
// Verify that getdents64 works with smaller buffer and yields same results as above
107+
std::vector<test_dirent64> dirents_2;
108+
test_getdents64("/proc", SMALL_BUF_SIZE, dirents_2);
109+
110+
assert(std::equal(dirents_1.begin(), dirents_1.end(), dirents_2.begin()));
111+
}

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