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util: introduce negative_errno()

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Imagine a constructor like this:

        int object_new(void **out) {
                void *my_object;
                int r;

                ...
                r = ioctl(...);
                if (r < 0)
                        return -errno;
                ...

                *out = my_object;
                return 0;
        }

We have a lot of those in systemd. If you now call those, gcc might inline
the call and optimize it. However, gcc cannot know that "errno" is
negative if "r" is. Therefore, a caller like this will produce warnings:

        r = object_new(&obj);
        if (r < 0)
                return r;

        obj->xyz = "foobar";

In case the ioctl in the constructor fails, gcc might assume "errno" is 0
and thus the error-handling is not triggered. Therefore, "obj" is
uninitialized, but accessed. Gcc will warn about that.

The new negative_errno() helper can be used to mitigate those warnings.
The helper is guaranteed to return a negative integer. Furthermore, it
spills out runtime warnings if "errno" is non-negative.

Instead of returning "-errno", you can use:
        return negative_errno();

gcc will no longer assume that this can return >=0, thus, it will not warn
about it.

Use this new helper in libsystemd-terminal to fix some grdev-drm warnings.
This commit is contained in:
David Herrmann 2014-11-03 18:23:28 +01:00
parent e6c019026b
commit 44dd2c6e86
2 changed files with 13 additions and 4 deletions
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8
src/libsystemd-terminal/grdev-drm.c
View file

@ -1393,7 +1393,7 @@ static int grdrm_fb_new(grdrm_fb **out, grdrm_card *card, const struct drm_mode_
r = ioctl(card->fd, DRM_IOCTL_MODE_CREATE_DUMB, &create_dumb);
if (r < 0) {
r = -errno;
r = negative_errno();
log_debug("grdrm: %s: cannot create dumb buffer %" PRIu32 "x%" PRIu32": %m",
card->base.name, fb->base.width, fb->base.height);
return r;
@ -1407,7 +1407,7 @@ static int grdrm_fb_new(grdrm_fb **out, grdrm_card *card, const struct drm_mode_
r = ioctl(card->fd, DRM_IOCTL_MODE_MAP_DUMB, &map_dumb);
if (r < 0) {
r = -errno;
r = negative_errno();
log_debug("grdrm: %s: cannot map dumb buffer %" PRIu32 "x%" PRIu32": %m",
card->base.name, fb->base.width, fb->base.height);
return r;
@ -1415,7 +1415,7 @@ static int grdrm_fb_new(grdrm_fb **out, grdrm_card *card, const struct drm_mode_
fb->base.maps[0] = mmap(0, fb->sizes[0], PROT_WRITE, MAP_SHARED, card->fd, map_dumb.offset);
if (fb->base.maps[0] == MAP_FAILED) {
r = -errno;
r = negative_errno();
log_debug("grdrm: %s: cannot memory-map dumb buffer %" PRIu32 "x%" PRIu32": %m",
card->base.name, fb->base.width, fb->base.height);
return r;
@ -1433,7 +1433,7 @@ static int grdrm_fb_new(grdrm_fb **out, grdrm_card *card, const struct drm_mode_
r = ioctl(card->fd, DRM_IOCTL_MODE_ADDFB2, &add_fb);
if (r < 0) {
r = -errno;
r = negative_errno();
log_debug("grdrm: %s: cannot add framebuffer %" PRIu32 "x%" PRIu32": %m",
card->base.name, fb->base.width, fb->base.height);
return r;

9
src/shared/util.h
View file

@ -794,6 +794,15 @@ static inline void _reset_errno_(int *saved_errno) {
#define PROTECT_ERRNO _cleanup_(_reset_errno_) __attribute__((unused)) int _saved_errno_ = errno
static inline int negative_errno(void) {
/* This helper should be used to shut up gcc if you know 'errno' is
* negative. Instead of "return -errno;", use "return negative_errno();"
* It will suppress bogus gcc warnings in case it assumes 'errno' might
* be 0 and thus the caller's error-handling might not be triggered. */
assert_return(errno > 0, -EINVAL);
return -errno;
}
struct _umask_struct_ {
mode_t mask;
bool quit;