Messages in this thread | | | Subject | Re: [BUG] lseek on /proc/meminfo is broken in 4.19.59 maybe due to commit 1f4aace60b0e ("fs/seq_file.c: simplify seq_file iteration code and interface") | From | Sergei Turchanov <> | Date | Fri, 2 Aug 2019 11:40:02 +1000 |
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Hello!
Yes, your patch fixed this bug. Thank you very much!
With best regards, Sergei.
On 01.08.2019 19:14, NeilBrown wrote: > On Thu, Aug 01 2019, Sergei Turchanov wrote: > >> Hello! >> >> [ >> As suggested in previous discussion this behavior may be caused by your >> commit 1f4aace60b0e ("fs/seq_file.c: simplify seq_file iteration code and interface") >> ] > Yes.... I think I can see what happened. > removing: > - if (!m->count) { > - m->from = 0; > - m->index++; > - } > > from seq_read meant that ->index didn't get updated in a case that it > needs to be. > > Please confirm that the following patch fixes the problem. > I think it is correct, but I need to look it over more carefully in the > morning, and see if I can explain why it is correct. > > Thanks for the report. > NeilBrown > > diff --git a/fs/seq_file.c b/fs/seq_file.c > index 04f09689cd6d..1600034a929b 100644 > --- a/fs/seq_file.c > +++ b/fs/seq_file.c > @@ -119,6 +119,7 @@ static int traverse(struct seq_file *m, loff_t offset) > } > if (seq_has_overflowed(m)) > goto Eoverflow; > + p = m->op->next(m, p, &m->index); > if (pos + m->count > offset) { > m->from = offset - pos; > m->count -= m->from; > @@ -126,7 +127,6 @@ static int traverse(struct seq_file *m, loff_t offset) > } > pos += m->count; > m->count = 0; > - p = m->op->next(m, p, &m->index); > if (pos == offset) > break; > } > > >> Original bug report: >> >> Seeking (to an offset within file size) in /proc/meminfo is broken in 4.19.59. It does seek to a desired position, but reading from that position returns the remainder of file and then a whole copy of file. This doesn't happen with /proc/vmstat or /proc/self/maps for example. >> >> Seeking did work correctly in kernel 4.14.47. So it seems something broke in the way. >> >> Background: this kind of access pattern (seeking to /proc/meminfo) is used by libvirt-lxc fuse driver for virtualized view of /proc/meminfo. So that /proc/meminfo is broken in guests when running kernel 4.19.x. >> >> > On 01.08.2019 17:11, Gao Xiang wrote: >>> Hi, >>> >>> I just took a glance, maybe due to >>> commit 1f4aace60b0e ("fs/seq_file.c: simplify seq_file iteration code and interface") >>> >>> I simply reverted it just now and it seems fine... but I haven't digged into this commit. >>> >>> Maybe you could Cc NeilBrown <neilb@suse.com> for some more advice and >>> I have no idea whether it's an expected behavior or not... >>> >>> Thanks, >>> Gao Xiang >>> >>> On 2019/8/1 14:16, Sergei Turchanov wrote: >> >> $ ./test /proc/meminfo 0 # Works as expected >> >> MemTotal: 394907728 kB >> MemFree: 173738328 kB >> ... >> DirectMap2M: 13062144 kB >> DirectMap1G: 390070272 kB >> >> ----------------------------------------------------------------------- >> >> $ ./test /proc/meminfo 1024 # returns a copy of file after the remainder >> >> Will seek to 1024 >> >> >> Data read at offset 1024 >> gePages: 0 kB >> ShmemHugePages: 0 kB >> ShmemPmdMapped: 0 kB >> HugePages_Total: 0 >> HugePages_Free: 0 >> HugePages_Rsvd: 0 >> HugePages_Surp: 0 >> Hugepagesize: 2048 kB >> Hugetlb: 0 kB >> DirectMap4k: 245204 kB >> DirectMap2M: 13062144 kB >> DirectMap1G: 390070272 kB >> MemTotal: 394907728 kB >> MemFree: 173738328 kB >> MemAvailable: 379989680 kB >> Buffers: 355812 kB >> Cached: 207216224 kB >> ... >> DirectMap2M: 13062144 kB >> DirectMap1G: 390070272 kB >> >> As you see, after "DirectMap1G:" line, a whole copy of /proc/meminfo returned by "read". >> >> Test program: >> >> #include <sys/types.h> >> #include <sys/stat.h> >> #include <unistd.h> >> #include <fcntl.h> >> #include <stdio.h> >> #include <stdlib.h> >> >> #define SIZE 1024 >> char buf[SIZE + 1]; >> >> int main(int argc, char *argv[]) { >> int fd; >> ssize_t rd; >> off_t ofs = 0; >> >> if (argc < 2) { >> printf("Usage: test <file> [<offset>]\n"); >> exit(1); >> } >> >> if (-1 == (fd = open(argv[1], O_RDONLY))) { >> perror("open failed"); >> exit(1); >> } >> >> if (argc > 2) { >> ofs = atol(argv[2]); >> } >> printf("Will seek to %ld\n", ofs); >> >> if (-1 == (lseek(fd, ofs, SEEK_SET))) { >> perror("lseek failed"); >> exit(1); >> } >> >> for (;; ofs += rd) { >> printf("\n\nData read at offset %ld\n", ofs); >> if (-1 == (rd = read(fd, buf, SIZE))) { >> perror("read failed"); >> exit(1); >> } >> buf[rd] = '\0'; >> printf(buf); >> if (rd < SIZE) { >> break; >> } >> } >> >> return 0; >> }
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