[PATCH] tmpfs: fix uninitialized return value in shmem_link

[PATCH] tmpfs: fix uninitialized return value in shmem_link

[Darrick J. Wong]

From: Darrick J. Wong <darrick.wong@oracle.com>

When we made the shmem_reserve_inode call in shmem_link conditional, we
forgot to update the declaration for ret so that it always has a known
value.  Dan Carpenter pointed out this deficiency in the original patch.

Fixes: "tmpfs: fix link accounting when a tmpfile is linked in"
Reported-by: dan.carpenter@oracle.com
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
 mm/shmem.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 0905215fb016..2c012eee133d 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2848,7 +2848,7 @@ static int shmem_create(struct inode *dir, struct dentry *dentry, umode_t mode,
 static int shmem_link(struct dentry *old_dentry, struct inode *dir, struct dentry *dentry)
 {
 	struct inode *inode = d_inode(old_dentry);
-	int ret;
+	int ret = 0;
 
 	/*
 	 * No ordinary (disk based) filesystem counts links as inodes;

Re: [PATCH] tmpfs: fix uninitialized return value in shmem_link

[Hugh Dickins]

On Thu, Feb 21, 2019 at 2:21 PM Darrick J. Wong <darrick.wong@oracle.com> wrote:
>
> From: Darrick J. Wong <darrick.wong@oracle.com>
>
> When we made the shmem_reserve_inode call in shmem_link conditional, we
> forgot to update the declaration for ret so that it always has a known
> value.  Dan Carpenter pointed out this deficiency in the original patch.
>
> Fixes: "tmpfs: fix link accounting when a tmpfile is linked in"
> Reported-by: dan.carpenter@oracle.com
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

Gosh, yes indeed: thanks Dan and Darrick, I'm very sorry for not noticing that.
Acked-by: Hugh Dickins <hughd@google.com>
(and sorry if this mail is garbled: it's from gmail, I cannot use
alpine at the moment).

> ---
>  mm/shmem.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 0905215fb016..2c012eee133d 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2848,7 +2848,7 @@ static int shmem_create(struct inode *dir, struct dentry *dentry, umode_t mode,
>  static int shmem_link(struct dentry *old_dentry, struct inode *dir, struct dentry *dentry)
>  {
>         struct inode *inode = d_inode(old_dentry);
> -       int ret;
> +       int ret = 0;
>
>         /*
>          * No ordinary (disk based) filesystem counts links as inodes;

[PATCH] tmpfs: fix uninitialized return value in shmem_link

[Hugh Dickins]

From: "Darrick J. Wong" <darrick.wong@oracle.com>

When we made the shmem_reserve_inode call in shmem_link conditional, we
forgot to update the declaration for ret so that it always has a known
value.  Dan Carpenter pointed out this deficiency in the original patch.

Fixes: 1062af920c07 ("tmpfs: fix link accounting when a tmpfile is linked in")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
---
 mm/shmem.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 0905215fb016..2c012eee133d 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2848,7 +2848,7 @@ static int shmem_create(struct inode *dir, struct dentry *dentry, umode_t mode,
 static int shmem_link(struct dentry *old_dentry, struct inode *dir, struct dentry *dentry)
 {
 	struct inode *inode = d_inode(old_dentry);
-	int ret;
+	int ret = 0;
 
 	/*
 	 * No ordinary (disk based) filesystem counts links as inodes;

Re: [PATCH] tmpfs: fix uninitialized return value in shmem_link

[Linus Torvalds]

On Fri, Feb 22, 2019 at 10:35 PM Hugh Dickins <hughd@google.com> wrote:
>
> When we made the shmem_reserve_inode call in shmem_link conditional, we
> forgot to update the declaration for ret so that it always has a known
> value.  Dan Carpenter pointed out this deficiency in the original patch.

Applied.

Side note: how come gcc didn't warn about this? Yes, we disable that
warning for some cases because of lots of false positives, but I
thought the *default* setup still had it.

Is it just that the goto ends up confusing gcc enough that it never notices?

                Linus

Re: [PATCH] tmpfs: fix uninitialized return value in shmem_link

[Hugh Dickins]

On Mon, 25 Feb 2019, Linus Torvalds wrote:
> On Fri, Feb 22, 2019 at 10:35 PM Hugh Dickins <hughd@google.com> wrote:
> >
> > When we made the shmem_reserve_inode call in shmem_link conditional, we
> > forgot to update the declaration for ret so that it always has a known
> > value.  Dan Carpenter pointed out this deficiency in the original patch.
> 
> Applied.

Thanks.  And I apologize for letting that slip through: Darrick sent
the patch fragment, I dressed it up, and more or less tricked him into
taking ownership of the bug, when it's I who should have been more careful.

But I'm glad it confirmed your rc8 instinct, rather than messing final :)

> 
> Side note: how come gcc didn't warn about this? Yes, we disable that
> warning for some cases because of lots of false positives, but I
> thought the *default* setup still had it.

I thought so too, and have been puzzled by it.  If I try removing the
initialization of inode from the next function, shmem_unlink(), I do
get the expected warning for that.

> 
> Is it just that the goto ends up confusing gcc enough that it never notices?

Since the goto route did have ret properly initialized, I don't see
why it might have been confusing, but what do I know...

I thought it might be because outside the goto route, ret was used
for nothing but the return value.  But that's disproved: I tried a
very silly "inode->i_flags = ret;" just after d_instantiate(),
and still no warning when ret is uninitialized.

Seems like a gcc bug? But I don't have a decent recent gcc to hand
to submit a proper report, hope someone else can shed light on it.

Hugh

Re: [PATCH] tmpfs: fix uninitialized return value in shmem_link

[Linus Torvalds]

[-- Attachment #1: Type: text/plain, Size: 2899 bytes --]

On Mon, Feb 25, 2019 at 12:34 PM Hugh Dickins <hughd@google.com> wrote:
>
> Seems like a gcc bug? But I don't have a decent recent gcc to hand
> to submit a proper report, hope someone else can shed light on it.

I don't have a _very_ recent gcc either, but with gcc-8.2.1 the
attached test-case gives me:

   [torvalds@i7 ~]$ gcc -O2 -S -Wall test.c

with no warning, and then

   [torvalds@i7 ~]$ gcc -O2 -S -Wall -DHIDE_PROBLEM test.c
   test.c: In function ‘shmem_link’:
   test.c:60:9: warning: ‘ret’ may be used uninitialized in this
function [-Wmaybe-uninitialized]
     return ret;
            ^~~

*does* show the expected warning.

So it is the presence of that

      if (ret) return ret;

that suppresses the warning.

What I *suspect* happens is

 (a) gcc sees that there is only one assignment to "ret"

 (b) in the same basic block as the assignment, there is a test
against "ret" being nonzero that goes out.

and what I think happens is that (a) causes gcc to consider that
assignment to be the defining assignment (which makes all kinds of
sense in an SSA world), and then (b) means that gcc decides that
clearly "ret" has to be zero in any case that doesn't go out due to
the if-test.

In fact, if I then look at the code generation, gcc will actually do
this (edited to be more legible):

        movl    (%rbx), %eax       <- load inode->i_nlink
        testl   %eax, %eax
        je      .L1
       ...
       ...
        call    d_instantiate
        xorl    %eax, %eax     <- explicitly zero 'ret'!
.L1:
        popq    %rbx
        popq    %rbp
        popq    %r12
        ret

so at least with my compiler, it *effectively* zeroed ret (in %rax)
anyway, and it all just _happened_ to get the right result even though
'ret' wasn't actually initialized.

Which is why it all worked just fine. And depending on how gcc works
internally, it really may not just be a random mistake of register
allocation, but really because gcc kind of _thought_ that 'ret' was
zero-initialized due to the combination of the one single assigment
and test for zero.

So it turns out that the patch to initialize to zero doesn't do
anything, probably for the same reason that gcc didn't warn about the
missing initialization. Gcc kind of added an initialization of its own
there.

I'm not entirely sure if any gcc developer would be interested in this
as a test-case, but I guess I can try to do a bugzilla.

Adding a few gcc people who have been on previous kernel gcc bugzilla
discussions, just in case they have something to add.

The gcc bugzilla is this:

    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89501

and I tried to make it be self-explanatory, but I wrote the bugzilla
in parallel with this email, and maybe there's some missing context
either there (or here).

                 Linus

[-- Attachment #2: test.c --]
[-- Type: text/x-c-code, Size: 1531 bytes --]

/*
 * Minimal fake declarations of "kernel" data types
 */
struct superblock;

struct inode {
	int i_nlink;
	int i_size;
	int i_ctime;
	int i_mtime;
	struct superblock *i_sb;
};

struct dentry {
	struct inode *d_inode;
};

#define d_inode(dentry) ((dentry)->d_inode)

extern int current_time(struct inode *);
extern void inc_nlink(struct inode *);
extern void ihold(struct inode *);
extern void dget(struct dentry *);
extern void d_instantiate(struct dentry *, struct inode *);
extern int shmem_reserve_inode(struct superblock *);

#define BOGO_DIRENT_SIZE 20

/*
 * The actual function where I'd have expected a warning
 * about "ret might be used uninitialized"
 */
int shmem_link(struct dentry *old_dentry, struct inode *dir,
		      struct dentry *dentry)
{
	struct inode *inode = d_inode(old_dentry);
	int ret;

	/*
	 * No ordinary (disk based) filesystem counts links as inodes;
	 * but each new link needs a new dentry, pinning lowmem, and
	 * tmpfs dentries cannot be pruned until they are unlinked.
	 * But if an O_TMPFILE file is linked into the tmpfs, the
	 * first link must skip that, to get the accounting right.
	 */
	if (inode->i_nlink) {
		ret = shmem_reserve_inode(inode->i_sb);
#ifndef HIDE_PROBLEM
		if (ret)
			return ret;
#endif
	}

	dir->i_size += BOGO_DIRENT_SIZE;
	inode->i_ctime = dir->i_ctime = dir->i_mtime = current_time(inode);
	inc_nlink(inode);
	ihold(inode);		/* New dentry reference */
	dget(dentry);		/* Extra pinning count for the created dentry */
	d_instantiate(dentry, inode);
	return ret;
}

Re: [PATCH] tmpfs: fix uninitialized return value in shmem_link

[Linus Torvalds]

On Mon, Feb 25, 2019 at 2:34 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Mon, Feb 25, 2019 at 12:34 PM Hugh Dickins <hughd@google.com> wrote:
> >
> > Seems like a gcc bug? But I don't have a decent recent gcc to hand
> > to submit a proper report, hope someone else can shed light on it.
>
> I don't have a _very_ recent gcc either [..]

Well, that was quick. Yup, it's considered a gcc bug.

Sadly, it's just a different version of a really old bug:

    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=18501

which goes back to 2004.

Which I guess means we should not expect this to be fixed in gcc any time soon.

The *good* news (I guess) is that if we have other situations with
that pattern, and that lack of warning, it really is because gcc will
have generated code as if it was initialized (to the value that we
tested it must have been in the one basic block where it *was*
initialized).

So it won't leak random kernel data, and with the common error
condition case (like in this example - checking that we didn't have an
error) it will actually end up doing the right thing.

Entirely by mistake, and without a warniing, but still.. It could have
been much worse. Basically at least for this pattern, "lack of
warning" ends up meaning "it got initialized to the expected value".

Of course, that's just gcc. I have no idea what llvm ends up doing.

               Linus

Re: [PATCH] tmpfs: fix uninitialized return value in shmem_link

[Qian Cai]

On 2/25/19 6:58 PM, Linus Torvalds wrote:
> On Mon, Feb 25, 2019 at 2:34 PM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>>
>> On Mon, Feb 25, 2019 at 12:34 PM Hugh Dickins <hughd@google.com> wrote:
>>>
>>> Seems like a gcc bug? But I don't have a decent recent gcc to hand
>>> to submit a proper report, hope someone else can shed light on it.
>>
>> I don't have a _very_ recent gcc either [..]
> 
> Well, that was quick. Yup, it's considered a gcc bug.
> 
> Sadly, it's just a different version of a really old bug:
> 
>     https://gcc.gnu.org/bugzilla/show_bug.cgi?id=18501
> 
> which goes back to 2004.
> 
> Which I guess means we should not expect this to be fixed in gcc any time soon.
> 
> The *good* news (I guess) is that if we have other situations with
> that pattern, and that lack of warning, it really is because gcc will
> have generated code as if it was initialized (to the value that we
> tested it must have been in the one basic block where it *was*
> initialized).
> 
> So it won't leak random kernel data, and with the common error
> condition case (like in this example - checking that we didn't have an
> error) it will actually end up doing the right thing.
> 
> Entirely by mistake, and without a warniing, but still.. It could have
> been much worse. Basically at least for this pattern, "lack of
> warning" ends up meaning "it got initialized to the expected value".
> 
> Of course, that's just gcc. I have no idea what llvm ends up doing.
> 

Clang 7.0:

# clang  -O2 -S -Wall /tmp/test.c
/tmp/test.c:46:6: warning: variable 'ret' is used uninitialized whenever 'if'
condition is false [-Wsometimes-uninitialized]
        if (inode->i_nlink) {
            ^~~~~~~~~~~~~~
/tmp/test.c:60:9: note: uninitialized use occurs here
        return ret;
               ^~~
/tmp/test.c:46:2: note: remove the 'if' if its condition is always true
        if (inode->i_nlink) {
        ^~~~~~~~~~~~~~~~~~~~
/tmp/test.c:37:9: note: initialize the variable 'ret' to silence this warning
        int ret;
               ^
                = 0
1 warning generated.

Re: [PATCH] tmpfs: fix uninitialized return value in shmem_link

[Linus Torvalds]

On Mon, Feb 25, 2019 at 4:03 PM Qian Cai <cai@lca.pw> wrote:
> >
> > Of course, that's just gcc. I have no idea what llvm ends up doing.
>
> Clang 7.0:
>
> # clang  -O2 -S -Wall /tmp/test.c
> /tmp/test.c:46:6: warning: variable 'ret' is used uninitialized whenever 'if'
> condition is false [-Wsometimes-uninitialized]

Ok, good.

Do we have any clang builds in any of the zero-day robot
infrastructure or something? Should we?

And maybe this was how Dan noticed the problem in the first place? Or
is it just because of his eagle-eyes?

                  Linus

Re: [PATCH] tmpfs: fix uninitialized return value in shmem_link

[Darrick J. Wong]

On Mon, Feb 25, 2019 at 04:07:12PM -0800, Linus Torvalds wrote:
> On Mon, Feb 25, 2019 at 4:03 PM Qian Cai <cai@lca.pw> wrote:
> > >
> > > Of course, that's just gcc. I have no idea what llvm ends up doing.
> >
> > Clang 7.0:
> >
> > # clang  -O2 -S -Wall /tmp/test.c
> > /tmp/test.c:46:6: warning: variable 'ret' is used uninitialized whenever 'if'
> > condition is false [-Wsometimes-uninitialized]
> 
> Ok, good.
> 
> Do we have any clang builds in any of the zero-day robot
> infrastructure or something? Should we?
> 
> And maybe this was how Dan noticed the problem in the first place? Or
> is it just because of his eagle-eyes?

He didn't say specifically how he found it, but I would guess he was
running smatch...?

--D

>                   Linus

Re: [PATCH] tmpfs: fix uninitialized return value in shmem_link

[Qian Cai]

On Mon, 2019-02-25 at 16:07 -0800, Linus Torvalds wrote:
> On Mon, Feb 25, 2019 at 4:03 PM Qian Cai <cai@lca.pw> wrote:
> > > 
> > > Of course, that's just gcc. I have no idea what llvm ends up doing.
> > 
> > Clang 7.0:
> > 
> > # clang  -O2 -S -Wall /tmp/test.c
> > /tmp/test.c:46:6: warning: variable 'ret' is used uninitialized whenever
> > 'if'
> > condition is false [-Wsometimes-uninitialized]
> 
> Ok, good.
> 
> Do we have any clang builds in any of the zero-day robot
> infrastructure or something? Should we?
> 
> And maybe this was how Dan noticed the problem in the first place? Or
> is it just because of his eagle-eyes?
> 

BTW, even clang is able to generate warnings in your sample code, it does not
generate any warnings when compiling the buggy shmem.o via "make CC=clang". Here
is the objdump for arm64 (with KASAN_SW_TAGS inline).

000000000000effc <shmem_link>:
{
    effc:       f81c0ff7        str     x23, [sp, #-64]!
    f000:       a90157f6        stp     x22, x21, [sp, #16]
    f004:       a9024ff4        stp     x20, x19, [sp, #32]
    f008:       a9037bfd        stp     x29, x30, [sp, #48]
    f00c:       9100c3fd        add     x29, sp, #0x30
    f010:       aa0203f3        mov     x19, x2
    f014:       aa0103f5        mov     x21, x1
    f018:       aa0003f4        mov     x20, x0
    f01c:       94000000        bl      0 <_mcount>
    f020:       91016280        add     x0, x20, #0x58
    f024:       d2c20017        mov     x23, #0x100000000000            //
#17592186044416
    f028:       b2481c08        orr     x8, x0, #0xff00000000000000
    f02c:       f2fdfff7        movk    x23, #0xefff, lsl #48
    f030:       d344fd08        lsr     x8, x8, #4
    f034:       38776909        ldrb    w9, [x8, x23]
    f038:       940017d5        bl      14f8c <OUTLINED_FUNCTION_11>
    f03c:       54000060        b.eq    f048 <shmem_link+0x4c>  // b.none
    f040:       7103fd1f        cmp     w8, #0xff
    f044:       54000981        b.ne    f174 <shmem_link+0x178>  // b.any
    f048:       f9400014        ldr     x20, [x0]
        if (inode->i_nlink) {
    f04c:       91010280        add     x0, x20, #0x40
    f050:       b2481c08        orr     x8, x0, #0xff00000000000000
    f054:       d344fd08        lsr     x8, x8, #4
    f058:       38776909        ldrb    w9, [x8, x23]
    f05c:       940017cc        bl      14f8c <OUTLINED_FUNCTION_11>
    f060:       54000060        b.eq    f06c <shmem_link+0x70>  // b.none
    f064:       7103fd1f        cmp     w8, #0xff
    f068:       540008a1        b.ne    f17c <shmem_link+0x180>  // b.any
    f06c:       b9400008        ldr     w8, [x0]
    f070:       34000148        cbz     w8, f098 <shmem_link+0x9c>
    f074:       940018fd        bl      15468 <OUTLINED_FUNCTION_1124>
                ret = shmem_reserve_inode(inode->i_sb);
    f078:       38776909        ldrb    w9, [x8, x23]
    f07c:       940017c4        bl      14f8c <OUTLINED_FUNCTION_11>
    f080:       54000060        b.eq    f08c <shmem_link+0x90>  // b.none
    f084:       7103fd1f        cmp     w8, #0xff
    f088:       540007e1        b.ne    f184 <shmem_link+0x188>  // b.any
    f08c:       f9400000        ldr     x0, [x0]
    f090:       97fffcf6        bl      e468 <shmem_reserve_inode>
                if (ret)
    f094:       35000660        cbnz    w0, f160 <shmem_link+0x164>
        dir->i_size += BOGO_DIRENT_SIZE;
    f098:       910122a0        add     x0, x21, #0x48
    f09c:       b2481c08        orr     x8, x0, #0xff00000000000000
    f0a0:       d344fd09        lsr     x9, x8, #4
    f0a4:       3877692a        ldrb    w10, [x9, x23]
    f0a8:       94001828        bl      15148 <OUTLINED_FUNCTION_193>
    f0ac:       54000060        b.eq    f0b8 <shmem_link+0xbc>  // b.none
    f0b0:       7103fd1f        cmp     w8, #0xff
    f0b4:       540006c1        b.ne    f18c <shmem_link+0x190>  // b.any
    f0b8:       38776929        ldrb    w9, [x9, x23]
    f0bc:       94001a4a        bl      159e4 <OUTLINED_FUNCTION_1131>
    f0c0:       54000060        b.eq    f0cc <shmem_link+0xd0>  // b.none
    f0c4:       7103fd1f        cmp     w8, #0xff
    f0c8:       54000661        b.ne    f194 <shmem_link+0x198>  // b.any
    f0cc:       f9000009        str     x9, [x0]
        inode->i_ctime = dir->i_ctime = dir->i_mtime = current_time(inode);
    f0d0:       aa1403e0        mov     x0, x20
    f0d4:       910182b6        add     x22, x21, #0x60
    f0d8:       94000000        bl      0 <current_time>
    f0dc:       b2481ec9        orr     x9, x22, #0xff00000000000000
    f0e0:       d344fd29        lsr     x9, x9, #4

Re: [PATCH] tmpfs: fix uninitialized return value in shmem_link

[Qian Cai]

On Wed, 2019-02-27 at 09:09 -0500, Qian Cai wrote:
> On Mon, 2019-02-25 at 16:07 -0800, Linus Torvalds wrote:
> > On Mon, Feb 25, 2019 at 4:03 PM Qian Cai <cai@lca.pw> wrote:
> > > > 
> > > > Of course, that's just gcc. I have no idea what llvm ends up doing.
> > > 
> > > Clang 7.0:
> > > 
> > > # clang  -O2 -S -Wall /tmp/test.c
> > > /tmp/test.c:46:6: warning: variable 'ret' is used uninitialized whenever
> > > 'if'
> > > condition is false [-Wsometimes-uninitialized]
> > 
> > Ok, good.
> > 
> > Do we have any clang builds in any of the zero-day robot
> > infrastructure or something? Should we?
> > 
> > And maybe this was how Dan noticed the problem in the first place? Or
> > is it just because of his eagle-eyes?
> > 
> 
> BTW, even clang is able to generate warnings in your sample code, it does not
> generate any warnings when compiling the buggy shmem.o via "make CC=clang".
> Here is the objdump for arm64 (with KASAN_SW_TAGS inline).
> 

Ah, thanks to the commit 6e8d666e9253 ("Disable "maybe-uninitialized" warning
globally"), it will no longer generate this type of warnings until using "make
W=1" due to the commit a76bcf557ef4 ("Kbuild: enable -Wmaybe-uninitialized
warning for 'make W=1'"). Anyway, the generated code is the same using clang
with and without this patch.

    d_instantiate(dentry, inode);
4eec:       94000000        bl      0 <d_instantiate>
            ret = shmem_reserve_inode(inode->i_sb);
4ef0:       2a1f03e0        mov     w0, wzr             <---- ret = 0
    return ret;

Re: [PATCH] tmpfs: fix uninitialized return value in shmem_link

[Nathan Chancellor]

On Wed, Feb 27, 2019 at 09:09:40AM -0500, Qian Cai wrote:
> On Mon, 2019-02-25 at 16:07 -0800, Linus Torvalds wrote:
> > On Mon, Feb 25, 2019 at 4:03 PM Qian Cai <cai@lca.pw> wrote:
> > > > 
> > > > Of course, that's just gcc. I have no idea what llvm ends up doing.
> > > 
> > > Clang 7.0:
> > > 
> > > # clang  -O2 -S -Wall /tmp/test.c
> > > /tmp/test.c:46:6: warning: variable 'ret' is used uninitialized whenever
> > > 'if'
> > > condition is false [-Wsometimes-uninitialized]
> > 
> > Ok, good.
> > 
> > Do we have any clang builds in any of the zero-day robot
> > infrastructure or something? Should we?
> > 
> > And maybe this was how Dan noticed the problem in the first place? Or
> > is it just because of his eagle-eyes?
> > 
> 
> BTW, even clang is able to generate warnings in your sample code, it does not
> generate any warnings when compiling the buggy shmem.o via "make CC=clang". Here

Unfortunately, scripts/Kbuild.extrawarn disables -Wuninitialized for
Clang, which also disables -Wsometimes-uninitialized:

https://github.com/ClangBuiltLinux/linux/issues/381
https://clang.llvm.org/docs/DiagnosticsReference.html#wuninitialized

I'm going to be sending out patches to fix the warnings found with it
then enable it going forward so that things like this get caught.

Nathan

> is the objdump for arm64 (with KASAN_SW_TAGS inline).
> 
> 000000000000effc <shmem_link>:
> {
>     effc:       f81c0ff7        str     x23, [sp, #-64]!
>     f000:       a90157f6        stp     x22, x21, [sp, #16]
>     f004:       a9024ff4        stp     x20, x19, [sp, #32]
>     f008:       a9037bfd        stp     x29, x30, [sp, #48]
>     f00c:       9100c3fd        add     x29, sp, #0x30
>     f010:       aa0203f3        mov     x19, x2
>     f014:       aa0103f5        mov     x21, x1
>     f018:       aa0003f4        mov     x20, x0
>     f01c:       94000000        bl      0 <_mcount>
>     f020:       91016280        add     x0, x20, #0x58
>     f024:       d2c20017        mov     x23, #0x100000000000            //
> #17592186044416
>     f028:       b2481c08        orr     x8, x0, #0xff00000000000000
>     f02c:       f2fdfff7        movk    x23, #0xefff, lsl #48
>     f030:       d344fd08        lsr     x8, x8, #4
>     f034:       38776909        ldrb    w9, [x8, x23]
>     f038:       940017d5        bl      14f8c <OUTLINED_FUNCTION_11>
>     f03c:       54000060        b.eq    f048 <shmem_link+0x4c>  // b.none
>     f040:       7103fd1f        cmp     w8, #0xff
>     f044:       54000981        b.ne    f174 <shmem_link+0x178>  // b.any
>     f048:       f9400014        ldr     x20, [x0]
>         if (inode->i_nlink) {
>     f04c:       91010280        add     x0, x20, #0x40
>     f050:       b2481c08        orr     x8, x0, #0xff00000000000000
>     f054:       d344fd08        lsr     x8, x8, #4
>     f058:       38776909        ldrb    w9, [x8, x23]
>     f05c:       940017cc        bl      14f8c <OUTLINED_FUNCTION_11>
>     f060:       54000060        b.eq    f06c <shmem_link+0x70>  // b.none
>     f064:       7103fd1f        cmp     w8, #0xff
>     f068:       540008a1        b.ne    f17c <shmem_link+0x180>  // b.any
>     f06c:       b9400008        ldr     w8, [x0]
>     f070:       34000148        cbz     w8, f098 <shmem_link+0x9c>
>     f074:       940018fd        bl      15468 <OUTLINED_FUNCTION_1124>
>                 ret = shmem_reserve_inode(inode->i_sb);
>     f078:       38776909        ldrb    w9, [x8, x23]
>     f07c:       940017c4        bl      14f8c <OUTLINED_FUNCTION_11>
>     f080:       54000060        b.eq    f08c <shmem_link+0x90>  // b.none
>     f084:       7103fd1f        cmp     w8, #0xff
>     f088:       540007e1        b.ne    f184 <shmem_link+0x188>  // b.any
>     f08c:       f9400000        ldr     x0, [x0]
>     f090:       97fffcf6        bl      e468 <shmem_reserve_inode>
>                 if (ret)
>     f094:       35000660        cbnz    w0, f160 <shmem_link+0x164>
>         dir->i_size += BOGO_DIRENT_SIZE;
>     f098:       910122a0        add     x0, x21, #0x48
>     f09c:       b2481c08        orr     x8, x0, #0xff00000000000000
>     f0a0:       d344fd09        lsr     x9, x8, #4
>     f0a4:       3877692a        ldrb    w10, [x9, x23]
>     f0a8:       94001828        bl      15148 <OUTLINED_FUNCTION_193>
>     f0ac:       54000060        b.eq    f0b8 <shmem_link+0xbc>  // b.none
>     f0b0:       7103fd1f        cmp     w8, #0xff
>     f0b4:       540006c1        b.ne    f18c <shmem_link+0x190>  // b.any
>     f0b8:       38776929        ldrb    w9, [x9, x23]
>     f0bc:       94001a4a        bl      159e4 <OUTLINED_FUNCTION_1131>
>     f0c0:       54000060        b.eq    f0cc <shmem_link+0xd0>  // b.none
>     f0c4:       7103fd1f        cmp     w8, #0xff
>     f0c8:       54000661        b.ne    f194 <shmem_link+0x198>  // b.any
>     f0cc:       f9000009        str     x9, [x0]
>         inode->i_ctime = dir->i_ctime = dir->i_mtime = current_time(inode);
>     f0d0:       aa1403e0        mov     x0, x20
>     f0d4:       910182b6        add     x22, x21, #0x60
>     f0d8:       94000000        bl      0 <current_time>
>     f0dc:       b2481ec9        orr     x9, x22, #0xff00000000000000
>     f0e0:       d344fd29        lsr     x9, x9, #4
>