Re: [PATCH] refs: make sure we never pass NULL to hashcpy

[Junio C Hamano]

Michael Haggerty  writes:

> So `ref_transaction_update()` *does* need to set or clear the `HAVE_NEW`
> and `HAVE_OLD` bits as I sketched, to impedance-match between the two
> conventions.

OK, so ignoring HAVE_NEW/HAVE_OLD bits that the callers of
ref_transaction_update() may set in flags, and having
ref_transaction_update() compute these bits based on new/old_sha1
pointers from scratch, would be the right thing to do.

IOW

flags &= ~(REF_HAVE_NEW|REF_HAVE_OLD);
if (new_sha1)
flags |= REF_HAVE_NEW;
if (old_sha1)
flags |= REF_HAVE_OLD;

and your earlier "Does the warning go away if you change the line
to" does essentially the same thing.

> It's a shame how much time we've wasted discussing this. Maybe the code
> is trying to be too clever/efficient and needs a rethink.

It might be the case, but I do not know what to blame is "the two
conventions", an over-eager compiler, or a confused commenter on the
thread (that's me), though ;-).

Re: [PATCH] refs: make sure we never pass NULL to hashcpy

[Michael Haggerty]

On 09/08/2017 02:46 AM, Junio C Hamano wrote:
> Michael Haggerty  writes:
> 
>> I did just realize one thing: `ref_transaction_update()` takes `flags`
>> as an argument and alters it using
>>
>>> flags |= (new_sha1 ? REF_HAVE_NEW : 0) | (old_sha1 ? REF_HAVE_OLD : 
>>> 0);
>>
>> Perhaps gcc is *more* intelligent than we give it credit for, and is
>> actually worried that the `flags` argument passed in by the caller
>> might *already* have one of these bits set. In that case
>> `ref_transaction_add_update()` would indeed be called incorrectly.
>> Does the warning go away if you change that line to
>>
>>> if (new_sha1)
>>> flags |=REF_HAVE_NEW;
>>> else
>>> flags &= ~REF_HAVE_NEW;
>>> if (old_sha1)
>>> flags |=REF_HAVE_OLD;
>>> else
>>> flags &= ~REF_HAVE_OLD;
>>
>> ? This might be a nice change to have anyway, to isolate
>> `ref_transaction_update()` from mistakes by its callers.
> 
> I understand "drop HAVE_NEW bit if new_sha1 is NULL" part, but not
> the other side "add HAVE_NEW if new_SHA1 is not NULL"---doesn't the
> NEW/OLD flag exist exactly because some callers pass the address of
> an embedded oid.hash[] or null_sha1, instead of NULL, when one side 
> does not exist?  So new|old being NULL is a definite signal that we
> need to drop HAVE_NEW|OLD, but the reverse may not be true, no?  Is
> it OK to overwrite null_sha1[] that is passed from some codepaths?
> 
> ref_transaction_create and _delete pass null_sha1 on the missing
> side, while ref_transaction_verify passes NULL, while calling
> _update().  Should this distinction affect how _add_update() gets
> called?

There are two functions under discussion:

* `ref_transaction_add_update()` is the low-level, private function that
uses the `HAVE_{NEW,OLD}` bits to decide what to do.

* `ref_transaction_update()` (like
`ref_transaction_{create,delete,verify}()`) are public functions that
ignore the `HAVE_{NEW,OLD}` bits and base their behavior on whether
`new_sha1` and `old_sha1` are NULL.

Each of these functions has to support three possibilities for its SHA-1
arguments:

1. The SHA-1 is provided and not `null_sha1`—in this case it must match
the old value (if `old_sha1`) or it is the value to be set as the new
value (if `new_sha1`).

2. The SHA-1 is provided and is equal to `null_sha1`—in this case the
reference must not already exist (if `old_sha1` is `null_sha1`) or it
will be deleted (if `new_sha1` is `null_sha1`).

3. The SHA-1 is not provided at all—in this case the old value is
ignored (if `old_sha1` is not provided) or the reference is left
unchanged (if `new_sha1` is not provided).

Much of the current confusion stems because
`ref_transaction_add_update()` encodes the third condition using the
`REF_HAVE_*` bits, whereas `ref_transaction_update()` and its friends
encode the third condition by setting `old_sha1` or `new_sha1` to `NULL`.

So `ref_transaction_update()` *does* need to set or clear the `HAVE_NEW`
and `HAVE_OLD` bits as I sketched, to impedance-match between the two
conventions.

It's a shame how much time we've wasted discussing this. Maybe the code
is trying to be too clever/efficient and needs a rethink.

Michael

Re: [PATCH] refs: make sure we never pass NULL to hashcpy

[Junio C Hamano]

Michael Haggerty  writes:

> I did just realize one thing: `ref_transaction_update()` takes `flags`
> as an argument and alters it using
>
>> flags |= (new_sha1 ? REF_HAVE_NEW : 0) | (old_sha1 ? REF_HAVE_OLD : 
>> 0);
>
> Perhaps gcc is *more* intelligent than we give it credit for, and is
> actually worried that the `flags` argument passed in by the caller
> might *already* have one of these bits set. In that case
> `ref_transaction_add_update()` would indeed be called incorrectly.
> Does the warning go away if you change that line to
>
>> if (new_sha1)
>> flags |=REF_HAVE_NEW;
>> else
>> flags &= ~REF_HAVE_NEW;
>> if (old_sha1)
>> flags |=REF_HAVE_OLD;
>> else
>> flags &= ~REF_HAVE_OLD;
>
> ? This might be a nice change to have anyway, to isolate
> `ref_transaction_update()` from mistakes by its callers.

I understand "drop HAVE_NEW bit if new_sha1 is NULL" part, but not
the other side "add HAVE_NEW if new_SHA1 is not NULL"---doesn't the
NEW/OLD flag exist exactly because some callers pass the address of
an embedded oid.hash[] or null_sha1, instead of NULL, when one side 
does not exist?  So new|old being NULL is a definite signal that we
need to drop HAVE_NEW|OLD, but the reverse may not be true, no?  Is
it OK to overwrite null_sha1[] that is passed from some codepaths?

ref_transaction_create and _delete pass null_sha1 on the missing
side, while ref_transaction_verify passes NULL, while calling
_update().  Should this distinction affect how _add_update() gets
called?

Re: [PATCH] refs: make sure we never pass NULL to hashcpy

[Thomas Gummerer]

On 09/07, Michael Haggerty wrote:
> On Wed, Sep 6, 2017 at 3:26 AM, Junio C Hamano  wrote:
> > Thomas Gummerer  writes:
> >
> >> gcc on arch linux (version 7.1.1) warns that a NULL argument is passed
> >> as the second parameter of memcpy.
> >> [...]
> >
> > It is hugely annoying to see a halfway-intelligent compiler forces
> > you to add such pointless asserts.
> >
> > The only way the compiler could error on this is by inferring the
> > fact that new_sha1/old_sha1 could be NULL by looking at the callsite
> > in ref_transaction_update() where these are used as conditionals to
> > set HAVE_NEW/HAVE_OLD that are passed.  Even if the compiler were
> > doing the whole-program analysis, the other two callsites of the
> > function pass the address of oid.hash[] in an oid structure so it
> > should know these won't be NULL.
> >
> > [...]
> >
> > I wonder if REF_HAVE_NEW/REF_HAVE_OLD are really needed in these
> > codepaths, though.  Perhaps we can instead declare !!new_sha1 means
> > we have the new side and rewrite the above part to
> >
> > if (new_sha1)
> > hashcpy(update->new_oid.hash, new_sha1);
> >
> > without an extra and totally pointless assert()?
> 
> The ultimate reason for those flags is that `struct ref_update` embeds
> `new_oid` and `old_oid` directly in the struct, so there is no way to
> set it to "NULL". (The `is_null_sha1` value is used for a different
> purpose.) So those flags keep track of whether the corresponding value
> is specified or absent.
> 
> Four of the five callers of `ref_transaction_add_update()` are
> constructing a new `ref_update` from an old one. They currently don't
> have to look into `flags`; they just pass it on (possibly changing a
> bit or two). Implementing your proposal would oblige those callers to
> change from something like

Thanks for the explanation!

> > new_update = ref_transaction_add_update(
> > transaction, "HEAD",
> > update->flags | REF_LOG_ONLY | REF_NODEREF,
> > update->new_oid.hash, update->old_oid.hash,
> > update->msg);
> 
> to
> 
> > new_update = ref_transaction_add_update(
> > transaction, "HEAD",
> > update->flags | REF_LOG_ONLY | REF_NODEREF,
> > (update->flags & REF_HAVE_NEW) ? update->new_oid.hash : NULL,
> > (update->flags & REF_HAVE_OLD) ? update->old_oid.hash : NULL,
> > update->msg);
> 
> It's not the end of the world, but it's annoying.
> `ref_transaction_add_update()` was meant to be a low-level,
> low-overhead way of allocating a `struct ref_update` and add it to a
> transaction.
> 
> Another solution (also annoying, but maybe a tad less so) would be to
> change the one iffy caller, `ref_transaction_update()`, to pass in a
> pointer to the null OID for `new_sha1` and `old_sha1` when the
> corresponding flags are turned off. That value would never be looked
> at, but it would hopefully reassure gcc.
> 
> I did just realize one thing: `ref_transaction_update()` takes `flags`
> as an argument and alters it using
> 
> > flags |= (new_sha1 ? REF_HAVE_NEW : 0) | (old_sha1 ? REF_HAVE_OLD : 
> > 0);
> 
> Perhaps gcc is *more* intelligent than we give it credit for, and is
> actually worried that the `flags` argument passed in by the caller
> might *already* have one of these bits set. In that case
> `ref_transaction_add_update()` would indeed be called incorrectly.
> Does the warning go away if you change that line to
> 
> > if (new_sha1)
> > flags |=REF_HAVE_NEW;
> > else
> > flags &= ~REF_HAVE_NEW;
> > if (old_sha1)
> > flags |=REF_HAVE_OLD;
> > else
> > flags &= ~REF_HAVE_OLD;
> 
> ?

Indeed that fixes it, great catch!  gcc is indeed smarter than we gave
it credit for, this makes a lot of sense.

Interestingly stripping away the flags fixes the compiler warning:

diff --git a/refs.c b/refs.c
index ba22f4acef..2e6871beac 100644
--- a/refs.c
+++ b/refs.c
@@ -921,6 +921,9 @@ int ref_transaction_update(struct ref_transaction 
*transaction,
return -1;
}
 
+   flags &= ~REF_HAVE_NEW;
+   flags &= ~REF_HAVE_OLD;
+
flags |= (new_sha1 ? REF_HAVE_NEW : 0) | (old_sha1 ? REF_HAVE_OLD : 0);
 
ref_transaction_add_update(transaction, refname, flags,

while checking that the flags are not there in the first place still
leaves the compiler warning (whether I use "die()" or just "return -1"
doesn't matter in this case):

diff --git a/refs.c b/refs.c
index ba22f4acef..62ff283755 100644
--- a/refs.c
+++ b/refs.c
@@ -921,6 +921,9 @@ int ref_transaction_update(struct ref_transaction 
*transaction,
return -1;
}
 
+   if ((flags & REF_HAVE_NEW) != 0 || (flags & REF_HAVE_OLD) != 0)
+   die("BUG: passed invalid flag to ref_transaction_update");
+
flags |= (new_sha1 ? REF_HAVE_NEW : 0) | (old_sha1 ? REF_HAVE_OLD : 0);
 

Re: [PATCH] refs: make sure we never pass NULL to hashcpy

[Michael Haggerty]

On Wed, Sep 6, 2017 at 3:26 AM, Junio C Hamano  wrote:
> Thomas Gummerer  writes:
>
>> gcc on arch linux (version 7.1.1) warns that a NULL argument is passed
>> as the second parameter of memcpy.
>> [...]
>
> It is hugely annoying to see a halfway-intelligent compiler forces
> you to add such pointless asserts.
>
> The only way the compiler could error on this is by inferring the
> fact that new_sha1/old_sha1 could be NULL by looking at the callsite
> in ref_transaction_update() where these are used as conditionals to
> set HAVE_NEW/HAVE_OLD that are passed.  Even if the compiler were
> doing the whole-program analysis, the other two callsites of the
> function pass the address of oid.hash[] in an oid structure so it
> should know these won't be NULL.
>
> [...]
>
> I wonder if REF_HAVE_NEW/REF_HAVE_OLD are really needed in these
> codepaths, though.  Perhaps we can instead declare !!new_sha1 means
> we have the new side and rewrite the above part to
>
> if (new_sha1)
> hashcpy(update->new_oid.hash, new_sha1);
>
> without an extra and totally pointless assert()?

The ultimate reason for those flags is that `struct ref_update` embeds
`new_oid` and `old_oid` directly in the struct, so there is no way to
set it to "NULL". (The `is_null_sha1` value is used for a different
purpose.) So those flags keep track of whether the corresponding value
is specified or absent.

Four of the five callers of `ref_transaction_add_update()` are
constructing a new `ref_update` from an old one. They currently don't
have to look into `flags`; they just pass it on (possibly changing a
bit or two). Implementing your proposal would oblige those callers to
change from something like

> new_update = ref_transaction_add_update(
> transaction, "HEAD",
> update->flags | REF_LOG_ONLY | REF_NODEREF,
> update->new_oid.hash, update->old_oid.hash,
> update->msg);

to

> new_update = ref_transaction_add_update(
> transaction, "HEAD",
> update->flags | REF_LOG_ONLY | REF_NODEREF,
> (update->flags & REF_HAVE_NEW) ? update->new_oid.hash : NULL,
> (update->flags & REF_HAVE_OLD) ? update->old_oid.hash : NULL,
> update->msg);

It's not the end of the world, but it's annoying.
`ref_transaction_add_update()` was meant to be a low-level,
low-overhead way of allocating a `struct ref_update` and add it to a
transaction.

Another solution (also annoying, but maybe a tad less so) would be to
change the one iffy caller, `ref_transaction_update()`, to pass in a
pointer to the null OID for `new_sha1` and `old_sha1` when the
corresponding flags are turned off. That value would never be looked
at, but it would hopefully reassure gcc.

I did just realize one thing: `ref_transaction_update()` takes `flags`
as an argument and alters it using

> flags |= (new_sha1 ? REF_HAVE_NEW : 0) | (old_sha1 ? REF_HAVE_OLD : 
> 0);

Perhaps gcc is *more* intelligent than we give it credit for, and is
actually worried that the `flags` argument passed in by the caller
might *already* have one of these bits set. In that case
`ref_transaction_add_update()` would indeed be called incorrectly.
Does the warning go away if you change that line to

> if (new_sha1)
> flags |=REF_HAVE_NEW;
> else
> flags &= ~REF_HAVE_NEW;
> if (old_sha1)
> flags |=REF_HAVE_OLD;
> else
> flags &= ~REF_HAVE_OLD;

? This might be a nice change to have anyway, to isolate
`ref_transaction_update()` from mistakes by its callers. For that
matter, one might want to be even more selective about what bits are
allowed in the `flags` argument to `ref_transaction_update()`'s
callers:

> flags &= REF_ALLOWED_FLAGS; /* value would need to be determined */

Michael

Re: [PATCH] refs: make sure we never pass NULL to hashcpy

[Thomas Gummerer]

On Wed, Sep 6, 2017 at 2:26 AM, Junio C Hamano  wrote:
> Thomas Gummerer  writes:
>
>> gcc on arch linux (version 7.1.1) warns that a NULL argument is passed
>> as the second parameter of memcpy.
>>
>> In file included from refs.c:5:0:
>> refs.c: In function ‘ref_transaction_verify’:
>> cache.h:948:2: error: argument 2 null where non-null expected 
>> [-Werror=nonnull]
>>   memcpy(sha_dst, sha_src, GIT_SHA1_RAWSZ);
>>   ^~~~
>> In file included from git-compat-util.h:165:0,
>>  from cache.h:4,
>>  from refs.c:5:
>> /usr/include/string.h:43:14: note: in a call to function ‘memcpy’ declared 
>> here
>>  extern void *memcpy (void *__restrict __dest, const void *__restrict __src,
>>   ^~
>> ...
>> diff --git a/refs.c b/refs.c
>> index ba22f4acef..d8c12a9c44 100644
>> --- a/refs.c
>> +++ b/refs.c
>> @@ -896,10 +896,14 @@ struct ref_update *ref_transaction_add_update(
>>
>>   update->flags = flags;
>>
>> - if (flags & REF_HAVE_NEW)
>> + if (flags & REF_HAVE_NEW) {
>> + assert(new_sha1);
>>   hashcpy(update->new_oid.hash, new_sha1);
>> - if (flags & REF_HAVE_OLD)
>> + }
>> + if (flags & REF_HAVE_OLD) {
>> + assert(old_sha1);
>>   hashcpy(update->old_oid.hash, old_sha1);
>> + }
>
> It is hugely annoying to see a halfway-intelligent compiler forces
> you to add such pointless asserts.
>
> The only way the compiler could error on this is by inferring the
> fact that new_sha1/old_sha1 could be NULL by looking at the callsite
> in ref_transaction_update() where these are used as conditionals to
> set HAVE_NEW/HAVE_OLD that are passed.  Even if the compiler were
> doing the whole-program analysis, the other two callsites of the
> function pass the address of oid.hash[] in an oid structure so it
> should know these won't be NULL.
>
> Or is the compiler being really dumb and triggering an error for
> every use of
>
> memcpy(dst, src, size);
>
> that must now be written as
>
> assert(src);
> memcpy(dst, src, size);
>
> ???  That would be doubly annoying

No, I think it can't quite deal with the flags that are passed in.
I'm on a different
machine today, so I can't actually check, but that's what I would
expect at least.

> I wonder if REF_HAVE_NEW/REF_HAVE_OLD are really needed in these
> codepaths, though.  Perhaps we can instead declare !!new_sha1 means
> we have the new side and rewrite the above part to
>
> if (new_sha1)
> hashcpy(update->new_oid.hash, new_sha1);
>
> without an extra and totally pointless assert()?

Yeah, that seems much nicer.  I'll try that and send a new a patch
(though I won't
get to it before tomorrow).  Thanks for the review.

>>   update->msg = xstrdup_or_null(msg);
>>   return update;
>>  }

Re: [PATCH] refs: make sure we never pass NULL to hashcpy

[Junio C Hamano]

Thomas Gummerer  writes:

> gcc on arch linux (version 7.1.1) warns that a NULL argument is passed
> as the second parameter of memcpy.
>
> In file included from refs.c:5:0:
> refs.c: In function ‘ref_transaction_verify’:
> cache.h:948:2: error: argument 2 null where non-null expected 
> [-Werror=nonnull]
>   memcpy(sha_dst, sha_src, GIT_SHA1_RAWSZ);
>   ^~~~
> In file included from git-compat-util.h:165:0,
>  from cache.h:4,
>  from refs.c:5:
> /usr/include/string.h:43:14: note: in a call to function ‘memcpy’ declared 
> here
>  extern void *memcpy (void *__restrict __dest, const void *__restrict __src,
>   ^~
> ...
> diff --git a/refs.c b/refs.c
> index ba22f4acef..d8c12a9c44 100644
> --- a/refs.c
> +++ b/refs.c
> @@ -896,10 +896,14 @@ struct ref_update *ref_transaction_add_update(
>  
>   update->flags = flags;
>  
> - if (flags & REF_HAVE_NEW)
> + if (flags & REF_HAVE_NEW) {
> + assert(new_sha1);
>   hashcpy(update->new_oid.hash, new_sha1);
> - if (flags & REF_HAVE_OLD)
> + }
> + if (flags & REF_HAVE_OLD) {
> + assert(old_sha1);
>   hashcpy(update->old_oid.hash, old_sha1);
> + }

It is hugely annoying to see a halfway-intelligent compiler forces
you to add such pointless asserts.

The only way the compiler could error on this is by inferring the
fact that new_sha1/old_sha1 could be NULL by looking at the callsite
in ref_transaction_update() where these are used as conditionals to
set HAVE_NEW/HAVE_OLD that are passed.  Even if the compiler were
doing the whole-program analysis, the other two callsites of the
function pass the address of oid.hash[] in an oid structure so it
should know these won't be NULL.

Or is the compiler being really dumb and triggering an error for
every use of

memcpy(dst, src, size);

that must now be written as

assert(src);
memcpy(dst, src, size);

???  That would be doubly annoying.

I wonder if REF_HAVE_NEW/REF_HAVE_OLD are really needed in these
codepaths, though.  Perhaps we can instead declare !!new_sha1 means
we have the new side and rewrite the above part to

if (new_sha1)
hashcpy(update->new_oid.hash, new_sha1);

without an extra and totally pointless assert()?

>   update->msg = xstrdup_or_null(msg);
>   return update;
>  }

[PATCH] refs: make sure we never pass NULL to hashcpy

[Thomas Gummerer]

gcc on arch linux (version 7.1.1) warns that a NULL argument is passed
as the second parameter of memcpy.

In file included from refs.c:5:0:
refs.c: In function ‘ref_transaction_verify’:
cache.h:948:2: error: argument 2 null where non-null expected [-Werror=nonnull]
  memcpy(sha_dst, sha_src, GIT_SHA1_RAWSZ);
  ^~~~
In file included from git-compat-util.h:165:0,
 from cache.h:4,
 from refs.c:5:
/usr/include/string.h:43:14: note: in a call to function ‘memcpy’ declared here
 extern void *memcpy (void *__restrict __dest, const void *__restrict __src,
  ^~

Tracking this error down, we can track it back to
ref_transaction_add_update.  where the call to hashcpy is however
protected by the flags that are passed in.

To make sure there's no code path where the wrong flags are passed in,
and to help the compiler realize that no NULL parameter is passed as
second argument to hashcpy, add asserts that this is indeed the case.

Signed-off-by: Thomas Gummerer 
---

This is based on top of ma/ts-cleanups, as that fixes another compiler
warning with gcc 7.1.1.

 refs.c | 8 ++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/refs.c b/refs.c
index ba22f4acef..d8c12a9c44 100644
--- a/refs.c
+++ b/refs.c
@@ -896,10 +896,14 @@ struct ref_update *ref_transaction_add_update(
 
update->flags = flags;
 
-   if (flags & REF_HAVE_NEW)
+   if (flags & REF_HAVE_NEW) {
+   assert(new_sha1);
hashcpy(update->new_oid.hash, new_sha1);
-   if (flags & REF_HAVE_OLD)
+   }
+   if (flags & REF_HAVE_OLD) {
+   assert(old_sha1);
hashcpy(update->old_oid.hash, old_sha1);
+   }
update->msg = xstrdup_or_null(msg);
return update;
 }
-- 
2.14.1.480.gb18f417b89