linux-user: fix readlink handling with magic exe symlink

The current code always returns the length of the path when it should
be returning the number of bytes it wrote to the output string.

Further, readlink is not supposed to append a NUL byte, but the current
snprintf logic will always do just that.

Even further, if you pass in a length of 0, you're suppoesd to get back
an error (EINVAL), but the current logic just returns 0.

Further still, if there was an error reading the symlink, we should not
go ahead and try to read the target buffer as it is garbage.

Simple test for the first two issues:
$ cat test.c
int main() {
    char buf[50];
    size_t len;
    for (len = 0; len < 10; ++len) {
        memset(buf, '!', sizeof(buf));
        ssize_t ret = readlink("/proc/self/exe", buf, len);
        buf[20] = '\0';
        printf("readlink(/proc/self/exe, {%s}, %zu) = %zi\n", buf, len, ret);
    }
    return 0;
}

Now compare the output of the native:
$ gcc test.c -o /tmp/x
$ /tmp/x
$ strace /tmp/x

With what qemu does:
$ armv7a-cros-linux-gnueabi-gcc test.c -o /tmp/x -static
$ qemu-arm /tmp/x
$ qemu-arm -strace /tmp/x

Signed-off-by: Mike Frysinger <vapier@chromium.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
1 file changed
tree: 70bf13634f0b10c3a3c2d9ed85fc3861aede7ac1
  1. audio/
  2. backends/
  3. block/
  4. bsd-user/
  5. default-configs/
  6. disas/
  7. docs/
  8. fpu/
  9. fsdev/
  10. gdb-xml/
  11. hw/
  12. include/
  13. libcacard/
  14. libdecnumber/
  15. linux-headers/
  16. linux-user/
  17. net/
  18. pc-bios/
  19. po/
  20. qapi/
  21. qga/
  22. qobject/
  23. qom/
  24. roms/
  25. scripts/
  26. slirp/
  27. stubs/
  28. sysconfigs/
  29. target-alpha/
  30. target-arm/
  31. target-cris/
  32. target-i386/
  33. target-lm32/
  34. target-m68k/
  35. target-microblaze/
  36. target-mips/
  37. target-moxie/
  38. target-openrisc/
  39. target-ppc/
  40. target-s390x/
  41. target-sh4/
  42. target-sparc/
  43. target-unicore32/
  44. target-xtensa/
  45. tcg/
  46. tests/
  47. trace/
  48. ui/
  49. util/
  50. .exrc
  51. .gitignore
  52. .gitmodules
  53. .mailmap
  54. .travis.yml
  55. aio-posix.c
  56. aio-win32.c
  57. arch_init.c
  58. async.c
  59. balloon.c
  60. block-migration.c
  61. block.c
  62. blockdev-nbd.c
  63. blockdev.c
  64. blockjob.c
  65. bt-host.c
  66. bt-vhci.c
  67. Changelog
  68. CODING_STYLE
  69. configure
  70. COPYING
  71. COPYING.LIB
  72. coroutine-gthread.c
  73. coroutine-sigaltstack.c
  74. coroutine-ucontext.c
  75. coroutine-win32.c
  76. cpu-exec.c
  77. cpus.c
  78. cputlb.c
  79. device-hotplug.c
  80. device_tree.c
  81. disas.c
  82. dma-helpers.c
  83. dump.c
  84. exec.c
  85. gdbstub.c
  86. HACKING
  87. hmp-commands.hx
  88. hmp.c
  89. hmp.h
  90. iohandler.c
  91. ioport.c
  92. iothread.c
  93. kvm-all.c
  94. kvm-stub.c
  95. LICENSE
  96. main-loop.c
  97. MAINTAINERS
  98. Makefile
  99. Makefile.objs
  100. Makefile.target
  101. memory.c
  102. memory_mapping.c
  103. migration-exec.c
  104. migration-fd.c
  105. migration-rdma.c
  106. migration-tcp.c
  107. migration-unix.c
  108. migration.c
  109. module-common.c
  110. monitor.c
  111. nbd.c
  112. numa.c
  113. os-posix.c
  114. os-win32.c
  115. page_cache.c
  116. qapi-schema.json
  117. qdev-monitor.c
  118. qdict-test-data.txt
  119. qemu-bridge-helper.c
  120. qemu-char.c
  121. qemu-coroutine-io.c
  122. qemu-coroutine-lock.c
  123. qemu-coroutine-sleep.c
  124. qemu-coroutine.c
  125. qemu-doc.texi
  126. qemu-file.c
  127. qemu-img-cmds.hx
  128. qemu-img.c
  129. qemu-img.texi
  130. qemu-io-cmds.c
  131. qemu-io.c
  132. qemu-log.c
  133. qemu-nbd.c
  134. qemu-nbd.texi
  135. qemu-options-wrapper.h
  136. qemu-options.h
  137. qemu-options.hx
  138. qemu-seccomp.c
  139. qemu-tech.texi
  140. qemu-timer.c
  141. qemu.nsi
  142. qemu.sasl
  143. qmp-commands.hx
  144. qmp.c
  145. qtest.c
  146. README
  147. rules.mak
  148. savevm.c
  149. softmmu_template.h
  150. spice-qemu-char.c
  151. tcg-runtime.c
  152. tci.c
  153. thread-pool.c
  154. thunk.c
  155. tpm.c
  156. trace-events
  157. translate-all.c
  158. translate-all.h
  159. user-exec.c
  160. VERSION
  161. version.rc
  162. vl.c
  163. vmstate.c
  164. xbzrle.c
  165. xen-common-stub.c
  166. xen-common.c
  167. xen-hvm-stub.c
  168. xen-hvm.c
  169. xen-mapcache.c