From 44dd2c6e861316d26a78848eb0f6b35bd3b82d4b Mon Sep 17 00:00:00 2001 From: David Herrmann Date: Mon, 3 Nov 2014 18:23:28 +0100 Subject: [PATCH] util: introduce negative_errno() 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. --- src/libsystemd-terminal/grdev-drm.c | 8 ++++---- src/shared/util.h | 9 +++++++++ 2 files changed, 13 insertions(+), 4 deletions(-) diff --git a/src/libsystemd-terminal/grdev-drm.c b/src/libsystemd-terminal/grdev-drm.c index dba6db269..48f8e9ce9 100644 --- a/src/libsystemd-terminal/grdev-drm.c +++ b/src/libsystemd-terminal/grdev-drm.c @@ -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; diff --git a/src/shared/util.h b/src/shared/util.h index e405b02a8..af589b670 100644 --- a/src/shared/util.h +++ b/src/shared/util.h @@ -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; -- 2.30.2