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Subject[RFC PATCH] 9p: case-insensitive host filesystems
Date
Now that 9p support for macOS hosts just landed in QEMU 7.0 and with support 
for Windows hosts on the horizon [1], the question is how to deal with case-
insensitive host filesystems, which are very common on those two systems?

I made some tests, e.g. trying to setup a 9p root fs Linux installation on a
macOS host as described in the QEMU HOWTO [2], which at a certain point causes
the debootstrap script to fail when trying to unpack the 'libpam-runtime'
package. That's because it would try to create this symlink:

/usr/share/man/man7/PAM.7.gz -> /usr/share/man/man7/pam.7.gz

which fails with EEXIST on a case-insensitive APFS. Unfortunately you can't
easily switch an existing APFS partition to case-sensitivity. It requires to
reformat the entire partition, loosing all your data, etc.

So I did a quick test with QEMU as outlined below, trying to simply let 9p
server "eat" EEXIST errors in such cases, but then I realized that most of the
time it would not even come that far, as Linux client would first send a
'Twalk' request to check whether target symlink entry already exists, and as
it gets a positive response from 9p server (again, due to case-insensitivity)
client would stop right there without even trying to send a 'Tsymlink'
request.

So maybe it's better to handle case-insensitivity entirely on client side?
I've read that some generic "case fold" code has landed in the Linux kernel
recently that might do the trick?

Should 9p server give a hint to 9p client that it's a case-insensitive fs? And
if yes, once per entire exported fs or rather for each directory (as there
might be submounts on host)?

[1] https://lore.kernel.org/all/20220408171013.912436-1-bmeng.cn@gmail.com/
[2] https://wiki.qemu.org/Documentation/9p_root_fs

---
hw/9pfs/9p-local.c | 54 ++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 54 insertions(+)

diff --git a/hw/9pfs/9p-local.c b/hw/9pfs/9p-local.c
index d42ce6d8b8..d6cb45c758 100644
--- a/hw/9pfs/9p-local.c
+++ b/hw/9pfs/9p-local.c
@@ -39,6 +39,10 @@
#endif
#endif
#include <sys/ioctl.h>
+#ifdef CONFIG_DARWIN
+#include <glib.h>
+#include <glib/gprintf.h>
+#endif

#ifndef XFS_SUPER_MAGIC
#define XFS_SUPER_MAGIC 0x58465342
@@ -57,6 +61,18 @@ typedef struct {
int mountfd;
} LocalData;

+#ifdef CONFIG_DARWIN
+
+/* Compare strings case-insensitive (assuming UTF-8 encoding). */
+static int p9_stricmp(const char *a, const char *b)
+{
+ g_autofree gchar *cia = g_utf8_casefold(a, -1);
+ g_autofree gchar *cib = g_utf8_casefold(b, -1);
+ return g_utf8_collate(cia, cib);
+}
+
+#endif
+
int local_open_nofollow(FsContext *fs_ctx, const char *path, int flags,
mode_t mode)
{
@@ -931,6 +947,25 @@ static int local_symlink(FsContext *fs_ctx, const char
*oldpath,
fs_ctx->export_flags & V9FS_SM_NONE) {
err = symlinkat(oldpath, dirfd, name);
if (err) {
+#if CONFIG_DARWIN
+ if (errno == EEXIST) {
+ printf(" -> symlinkat(oldpath='%s', dirfd=%d, name='%s') =
EEXIST\n", oldpath, dirfd, name);
+ }
+ if (errno == EEXIST &&
+ strcmp(oldpath, name) && !p9_stricmp(oldpath, name))
+ {
+ struct stat st1, st2;
+ const int cur_errno = errno;
+ if (!fstatat(dirfd, oldpath, &st1, AT_SYMLINK_NOFOLLOW) &&
+ !fstatat(dirfd, name, &st2, AT_SYMLINK_NOFOLLOW) &&
+ st1.st_dev == st2.st_dev && st1.st_ino == st2.st_ino)
+ {
+ printf(" -> iCASE SAME\n");
+ err = 0;
+ }
+ errno = cur_errno;
+ }
+#endif
goto out;
}
err = fchownat(dirfd, name, credp->fc_uid, credp->fc_gid,
@@ -983,6 +1018,25 @@ static int local_link(FsContext *ctx, V9fsPath *oldpath,

ret = linkat(odirfd, oname, ndirfd, name, 0);
if (ret < 0) {
+#if CONFIG_DARWIN
+ if (errno == EEXIST) {
+ printf(" -> linkat(odirfd=%d, oname='%s', ndirfd=%d, name='%s')
= EEXIST\n", odirfd, oname, ndirfd, name);
+ }
+ if (errno == EEXIST &&
+ strcmp(oname, name) && !p9_stricmp(oname, name))
+ {
+ struct stat st1, st2;
+ const int cur_errno = errno;
+ if (!fstatat(odirfd, oname, &st1, AT_SYMLINK_NOFOLLOW) &&
+ !fstatat(ndirfd, name, &st2, AT_SYMLINK_NOFOLLOW) &&
+ st1.st_dev == st2.st_dev && st1.st_ino == st2.st_ino)
+ {
+ printf(" -> iCASE SAME\n");
+ ret = 0;
+ }
+ errno = cur_errno;
+ }
+#endif
goto out_close;
}

--
2.32.0 (Apple Git-132)



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