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[209.85.220.65]) by mx.google.com with SMTPS id s2sor7412632qkg.82.2019.06.17.07.51.05 for (Google Transport Security); Mon, 17 Jun 2019 07:51:05 -0700 (PDT) Received-SPF: pass (google.com: domain of arndbergmann@gmail.com designates 209.85.220.65 as permitted sender) client-ip=209.85.220.65; Authentication-Results: mx.google.com; spf=pass (google.com: domain of arndbergmann@gmail.com designates 209.85.220.65 as permitted sender) smtp.mailfrom=arndbergmann@gmail.com X-Google-Smtp-Source: APXvYqzGHPK0SuAOiFmF36ZCL+V6bBx3pWjrebtOkfLE1KzmEr0hyrwu/CO4hn9QYjNX0GmKoAn1mJ5RYvF4ApuHWDg= X-Received: by 2002:ae9:e608:: with SMTP id z8mr80517080qkf.182.1560783065346; Mon, 17 Jun 2019 07:51:05 -0700 (PDT) MIME-Version: 1.0 References: <20190617121427.77565-1-arnd@arndb.de> <20190617141244.5x22nrylw7hodafp@pc636> In-Reply-To: From: Arnd Bergmann Date: Mon, 17 Jun 2019 16:50:48 +0200 Message-ID: Subject: Re: [BUG]: mm/vmalloc: uninitialized variable access in pcpu_get_vm_areas To: Uladzislau Rezki Cc: Roman Gushchin , Michal Hocko , Matthew Wilcox , Thomas Garnier , Oleksiy Avramchenko , Steven Rostedt , Joel Fernandes , Thomas Gleixner , Ingo Molnar , Tejun Heo , Andrew Morton , Linus Torvalds , Stephen Rothwell , Roman Penyaev , Rick Edgecombe , Andrey Ryabinin , Mike Rapoport , Linux-MM , Linux Kernel Mailing List Content-Type: text/plain; charset="UTF-8" X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Jun 17, 2019 at 4:44 PM Arnd Bergmann wrote: > On Mon, Jun 17, 2019 at 4:12 PM Uladzislau Rezki wrote: > > > > On Mon, Jun 17, 2019 at 02:14:11PM +0200, Arnd Bergmann wrote: > > > gcc points out some obviously broken code in linux-next > > > > > > mm/vmalloc.c: In function 'pcpu_get_vm_areas': > > > mm/vmalloc.c:991:4: error: 'lva' may be used uninitialized in this function [-Werror=maybe-uninitialized] > > > insert_vmap_area_augment(lva, &va->rb_node, > > > ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > > &free_vmap_area_root, &free_vmap_area_list); > > > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > > mm/vmalloc.c:916:20: note: 'lva' was declared here > > > struct vmap_area *lva; > > > ^~~ > > > > > > Remove the obviously broken code. This is almost certainly > > > not the correct solution, but it's what I have applied locally > > > to get a clean build again. > > > > > > Please fix this properly. > > > > > > > > > Please do not apply this. It will just break everything. > > As I wrote in my description, this was purely meant as a bug > report, not a patch to be applied. > > > As Roman pointed we can just set lva = NULL; in the beginning to make GCC happy. > > For some reason GCC decides that it can be used uninitialized, but that > > is not true. > > I got confused by the similarly named FL_FIT_TYPE/NE_FIT_TYPE > constants and misread this as only getting run in the case where it is > not initialized, but you are right that it always is initialized here. > > I see now that the actual cause of the warning is the 'while' loop in > augment_tree_propagate_from(). gcc is unable to keep track of > the state of the 'lva' variable beyond that and prints a bogus warning. I managed to un-confuse gcc-8 by turning the if/else if/else into a switch statement. If you all think this is an acceptable solution, I'll submit that after some more testing to ensure it addresses all configurations: diff --git a/mm/vmalloc.c b/mm/vmalloc.c index a9213fc3802d..5b7e50de008b 100644 --- a/mm/vmalloc.c +++ b/mm/vmalloc.c @@ -915,7 +915,8 @@ adjust_va_to_fit_type(struct vmap_area *va, { struct vmap_area *lva; - if (type == FL_FIT_TYPE) { + switch (type) { + case FL_FIT_TYPE: /* * No need to split VA, it fully fits. * @@ -925,7 +926,8 @@ adjust_va_to_fit_type(struct vmap_area *va, */ unlink_va(va, &free_vmap_area_root); kmem_cache_free(vmap_area_cachep, va); - } else if (type == LE_FIT_TYPE) { + break; + case LE_FIT_TYPE: /* * Split left edge of fit VA. * @@ -934,7 +936,8 @@ adjust_va_to_fit_type(struct vmap_area *va, * |-------|-------| */ va->va_start += size; - } else if (type == RE_FIT_TYPE) { + break; + case RE_FIT_TYPE: /* * Split right edge of fit VA. * @@ -943,7 +946,8 @@ adjust_va_to_fit_type(struct vmap_area *va, * |-------|-------| */ va->va_end = nva_start_addr; - } else if (type == NE_FIT_TYPE) { + break; + case NE_FIT_TYPE: /* * Split no edge of fit VA. * @@ -980,7 +984,8 @@ adjust_va_to_fit_type(struct vmap_area *va, * Shrink this VA to remaining size. */ va->va_start = nva_start_addr + size; - } else { + break; + default: return -1; } Arnd