A hard link can be made inside a bind mount or named volume

native/fs.h

@@ -1935,7 +1935,7 @@ static uint64_t fs_call_once(uint64_t call) {
         if(!fs_path_writable())error=linux_error(30);
         else if(where!=fs_where || where<0)error=linux_error(18);   /* EXDEV: never across a mount boundary */
         else if(!strcmp(from,".") || !strcmp(leaf,"."))error=linux_error(!strcmp(leaf,".")?17:1);
-        else if(old.layer==2)error=linux_error(2);
+        else if(old.canon[0] && old.layer==2)error=linux_error(2);   /* in the layers; in a bind mount (canon "") the host's linkat reports it */
         else if(ov_prepare(&w,&parent,leaf,OV_CREATE) || ov_prepare(&old,&source,from,OV_MODIFY))error=fs_error();  /* image layers: a lower file is copied up */
         else if(linkat(source,from,parent,leaf,0)<0)error=fs_error();
         else ov_created(&w,parent,leaf,0);

scripts/verify_overlay.py

@@ -4,7 +4,8 @@
 over one shared lower tree, the second run proving the first left the shared files
 alone); new files kept in memory (--upper-memory, native/upm.h) renamed while open
 over lower and upper files (tests/native/upm_guest.c), and what that run wrote
-back: each file whole, and no whiteout beside it; then, with alpine as the lower
+back: each file whole, and no whiteout beside it; hard links inside a bind mount
+(tests/native/mountlink_guest.c), never across its boundary; then, with alpine as the lower
 layer, busybox copying, archiving, renaming
 and removing image files, and a container trying to change the image's files every
 way it can (writes, chmod, rename, links, truncate, MAP_SHARED writes, sockets,
@@ -79,6 +80,39 @@ def runtimes():
             print(f'{binary}: skipped (not built)')
 
 
+def mount_link_runs(t):
+    """mountlink_guest.c on every built runtime, with and without the memory layer, bind
+    mounts at /vol and /vol2; then what it left: one file under its three names in the
+    first mount, nothing in the second, no whiteout in either."""
+    failed = 0
+    lower = t/'link-lower'
+    (lower/'low').mkdir(parents=True)
+    (lower/'low/f').write_text('f')
+    build(ROOT/'tests/native/mountlink_guest.c', t/'mountlink')
+    before = snapshot(lower)
+    for binary, runtime, extra in runtimes():
+        for memory in (False, True):
+            tag = f'{binary}-{int(memory)}'
+            upper, state, vol, vol2 = t/f'link-up-{tag}', t/f'link-state-{tag}', t/f'link-vol-{tag}', t/f'link-vol2-{tag}'
+            for d in (upper, state, vol, vol2):
+                d.mkdir()
+            more = ['--upper-memory', str(state/'upper-sync'), '--layers-gen', str(state/'gen')] if memory else []
+            r = subprocess.run([str(runtime), 'run', str(t/'mountlink'), '--root', str(upper), '--lower', str(lower), '--writable', '--bind', str(vol), '/vol', '--bind', str(vol2), '/vol2',
+                                '--storage-limit-mib', '0', '--max-steps', '2000000000', '--process-tree', '--scheduled-network', '--dynamic', *more, *extra],
+                               capture_output=True, timeout=300)
+            ok = r.returncode == 0 and r.stdout == b'mountlink PASS\n' and b'ERROR: AddressSanitizer' not in r.stderr
+            names = sorted(str(p.relative_to(vol)) for p in vol.rglob('*'))
+            ok = ok and names == ['a', 'b', 'd', 'd/c', 's'] and len({(vol/n).stat().st_ino for n in ('a', 'b', 'd/c')}) == 1 and (vol/'a').stat().st_nlink == 3
+            ok = ok and not os.listdir(vol2) and not os.path.lexists(upper/'x') and (upper/'low/g').is_file()
+            failed += not ok
+            print(f'{binary} mountlink_guest{" with the memory layer" if memory else ""}:', 'PASS' if ok else
+                  f'FAIL exit={r.returncode} {r.stdout!r} vol={names} vol2={os.listdir(vol2)} {r.stderr.decode()[-1500:]}')
+    same = snapshot(lower) == before
+    failed += not same
+    print('mountlink lower layer unchanged:', 'PASS' if same else 'FAIL')
+    return failed
+
+
 # upm_guest.c's lower tree, and what its run leaves in the upper layer: files written
 # back, and the whiteouts of the lower files it removed (and only those).
 UPM_LOWER = {'u/lower': 'old lower', 'u/lower2': 'old lower2', 'u/up': 'old up', 'u/app.log.1': 'old log', 'u/k': 'k', 'u/nr': 'nr',
@@ -203,6 +237,7 @@ def main():
         lower.mkdir()
         lower_tree(lower)
         build(ROOT/'tests/native/overlay_guest.c', t/'overlay')
+        failed += mount_link_runs(t)
         before = snapshot(lower)
         for binary, runtime, extra in runtimes():
             for run in (1, 2):                              # the second run sees the lower layer as the first did, with the lookup cache

tests/native/mountlink_guest.c

@@ -0,0 +1,65 @@
+/* Hard links inside a bind mount or named volume beside image layers (native/fs.h:
+   linkat): both names are one file (one inode, two links), by absolute path, by a
+   descriptor of the mount's directory, and through a symlink with AT_SYMLINK_FOLLOW; a
+   missing source is ENOENT, a taken name EEXIST, and a link that would leave the mount
+   (to another mount, to or from the container's layers) is EXDEV. /vol and /vol2 are
+   bind mounts, /low/f a file of the image. Run by scripts/verify_overlay.py, with and
+   without the memory layer, which then checks what the run left on the host. */
+typedef unsigned long U;
+static long call(U n,U a,U b,U c,U d,U e,U f) {
+    register U x8 __asm__("x8")=n,x0 __asm__("x0")=a,x1 __asm__("x1")=b;
+    register U x2 __asm__("x2")=c,x3 __asm__("x3")=d,x4 __asm__("x4")=e,x5 __asm__("x5")=f;
+    __asm__ volatile("svc #0":"+r"(x0):"r"(x8),"r"(x1),"r"(x2),"r"(x3),"r"(x4),"r"(x5):"memory","cc");return (long)x0;
+}
+#define C1(n,a) call(n,(U)(a),0,0,0,0,0)
+#define C3(n,a,b,c) call(n,(U)(a),(U)(b),(U)(c),0,0,0)
+#define C4(n,a,b,c,d) call(n,(U)(a),(U)(b),(U)(c),(U)(d),0,0)
+#define C5(n,a,b,c,d,e) call(n,(U)(a),(U)(b),(U)(c),(U)(d),(U)(e),0)
+#define AT (-100)
+#define ENOENT (-2)
+#define EEXIST (-17)
+#define EXDEV (-18)
+static void fail_at(int line) {
+    char text[40]="mountlink FAIL line ";unsigned n=20;char digits[8];unsigned k=0;
+    while(line && k<8) { digits[k++]=(char)('0'+line%10);line/=10; }
+    while(k)text[n++]=digits[--k];
+    text[n++]='\n';C3(64,1,text,n);C1(94,1);for(;;){}
+}
+#define CHECK(x) do { if(!(x))fail_at(__LINE__); } while(0)
+static unsigned long u64(const unsigned char *p) { unsigned long v=0;for(int i=7;i>=0;i--)v=v<<8|p[i];return v; }
+static unsigned char st[128];
+static long stat_of(const char *path) { return C4(79,AT,path,st,256); }          /* AT_SYMLINK_NOFOLLOW */
+static unsigned long ino(const char *path) { return stat_of(path)?0:u64(st+8); }
+static long links(const char *path) { return stat_of(path)?-1:(long)(st[20]|st[21]<<8); }
+static long put(const char *path) {
+    long fd=C4(56,AT,path,01101,0644);if(fd<0)return fd;                         /* O_WRONLY|O_CREAT|O_TRUNC */
+    long n=C3(64,fd,"x",1);C1(57,fd);return n;
+}
+static int has(const char *path) {
+    char c[2];long fd=C4(56,AT,path,0,0);if(fd<0)return 0;
+    long n=C3(63,fd,c,2);C1(57,fd);return n==1 && c[0]=='x';
+}
+static long link(const char *from,const char *to) { return C5(37,AT,from,AT,to,0); }
+
+void _start(void) {
+    CHECK(put("/vol/a")==1 && C3(34,AT,"/vol/d",0755)==0);
+    /* inside the mount: one file under both names */
+    CHECK(link("/vol/a","/vol/b")==0);
+    CHECK(ino("/vol/a") && ino("/vol/a")==ino("/vol/b") && links("/vol/a")==2 && has("/vol/b"));
+    CHECK(link("/vol/b","/vol/d/c")==0 && ino("/vol/d/c")==ino("/vol/a") && links("/vol/a")==3);
+    long dir=C4(56,AT,"/vol",040000,0);CHECK(dir>=0);                            /* O_DIRECTORY */
+    CHECK(C5(37,dir,"a",dir,"d/r",0)==0 && ino("/vol/d/r")==ino("/vol/a"));C1(57,dir);
+    CHECK(C3(36,"a",AT,"/vol/s")==0 && C5(37,AT,"/vol/s",AT,"/vol/t",0x400)==0 && ino("/vol/t")==ino("/vol/a"));   /* AT_SYMLINK_FOLLOW */
+    CHECK(C3(35,AT,"/vol/t",0)==0 && C3(35,AT,"/vol/d/r",0)==0 && links("/vol/a")==3 && has("/vol/a"));
+    CHECK(link("/vol/none","/vol/e")==ENOENT && stat_of("/vol/e")==ENOENT);
+    CHECK(link("/vol/a","/vol/b")==EEXIST && link("/vol/a","/vol/none/e")==ENOENT);
+    /* never across the mount's boundary */
+    CHECK(link("/vol/a","/vol2/x")==EXDEV && stat_of("/vol2/x")==ENOENT);
+    CHECK(link("/vol/a","/x")==EXDEV && link("/vol/a","/low/x")==EXDEV && stat_of("/x")==ENOENT && stat_of("/low/x")==ENOENT);
+    CHECK(link("/low/f","/vol/x")==EXDEV && put("/up")==1 && link("/up","/vol/x")==EXDEV && stat_of("/vol/x")==ENOENT);
+    /* the layers as before: an image file is linked (copied up), a missing one is not */
+    CHECK(link("/low/f","/low/g")==0 && ino("/low/g")==ino("/low/f") && links("/low/f")==2);
+    CHECK(link("/none","/y")==ENOENT && link("/low/none","/low/y")==ENOENT);
+    CHECK(links("/vol/a")==3);
+    C3(64,1,"mountlink PASS\n",15);C1(94,0);for(;;){}
+}