Excluindo um arquivo vazio, por que existem tantas chamadas sys?

Question

Tente criar um arquivo vazio e marque as operações envolvidas para excluí-lo. A lista é muito longa:

touch a
strace rm a
execve("/bin/rm", ["rm", "a"], [/* 20 vars */]) = 0
brk(0)                                  = 0x65f000
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd579b94000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=25132, ...}) = 0
mmap(NULL, 25132, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fd579b8d000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "7ELF
touch a
strace rm a
execve("/bin/rm", ["rm", "a"], [/* 20 vars */]) = 0
brk(0)                                  = 0x65f000
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd579b94000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=25132, ...}) = 0
mmap(NULL, 25132, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fd579b8d000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "7ELF%pre%%pre%%pre%%pre%%pre%%pre%%pre%%pre%%pre%%pre%>%pre%%pre%%pre%%pre%0%pre%%pre%%pre%%pre%%pre%"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=1840928, ...}) = 0
mmap(NULL, 3949248, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fd5795af000
mprotect(0x7fd57976a000, 2093056, PROT_NONE) = 0
mmap(0x7fd579969000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1ba000) = 0x7fd579969000
mmap(0x7fd57996f000, 17088, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7fd57996f000
close(3)                                = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd579b8c000
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd579b8a000
arch_prctl(ARCH_SET_FS, 0x7fd579b8a740) = 0
mprotect(0x7fd579969000, 16384, PROT_READ) = 0
mprotect(0x60d000, 4096, PROT_READ)     = 0
mprotect(0x7fd579b96000, 4096, PROT_READ) = 0
munmap(0x7fd579b8d000, 25132)           = 0
brk(0)                                  = 0x65f000
brk(0x680000)                           = 0x680000
open("/usr/lib/locale/locale-archive", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=2919792, ...}) = 0
mmap(NULL, 2919792, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fd5792e6000
close(3)                                = 0
ioctl(0, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
newfstatat(AT_FDCWD, "a", {st_mode=S_IFREG|0664, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
geteuid()                               = 1000
newfstatat(AT_FDCWD, "a", {st_mode=S_IFREG|0664, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
faccessat(AT_FDCWD, "a", W_OK)          = 0
unlinkat(AT_FDCWD, "a", 0)              = 0
lseek(0, 0, SEEK_CUR)                   = -1 ESPIPE (Illegal seek)
close(0)                                = 0
close(1)                                = 0
close(2)                                = 0
exit_group(0)                           = ?
+++ exited with 0 +++
%pre%%pre%%pre%%pre%%pre%%pre%%pre%%pre%%pre%>%pre%%pre%%pre%%pre%0%pre%%pre%%pre%%pre%%pre%"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=1840928, ...}) = 0
mmap(NULL, 3949248, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fd5795af000
mprotect(0x7fd57976a000, 2093056, PROT_NONE) = 0
mmap(0x7fd579969000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1ba000) = 0x7fd579969000
mmap(0x7fd57996f000, 17088, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7fd57996f000
close(3)                                = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd579b8c000
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd579b8a000
arch_prctl(ARCH_SET_FS, 0x7fd579b8a740) = 0
mprotect(0x7fd579969000, 16384, PROT_READ) = 0
mprotect(0x60d000, 4096, PROT_READ)     = 0
mprotect(0x7fd579b96000, 4096, PROT_READ) = 0
munmap(0x7fd579b8d000, 25132)           = 0
brk(0)                                  = 0x65f000
brk(0x680000)                           = 0x680000
open("/usr/lib/locale/locale-archive", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=2919792, ...}) = 0
mmap(NULL, 2919792, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fd5792e6000
close(3)                                = 0
ioctl(0, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
newfstatat(AT_FDCWD, "a", {st_mode=S_IFREG|0664, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
geteuid()                               = 1000
newfstatat(AT_FDCWD, "a", {st_mode=S_IFREG|0664, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
faccessat(AT_FDCWD, "a", W_OK)          = 0
unlinkat(AT_FDCWD, "a", 0)              = 0
lseek(0, 0, SEEK_CUR)                   = -1 ESPIPE (Illegal seek)
close(0)                                = 0
close(1)                                = 0
close(2)                                = 0
exit_group(0)                           = ?
+++ exited with 0 +++

Por que isso?

    

Answer 1

Todas as chamadas do sistema que aparecem acima da chamada close(3) são devidas ao carregamento dinâmico da biblioteca. Parece que também está verificando todas as bibliotecas vinculadas dinamicamente em cache.

As chamadas do sistema para realmente remover o arquivo começam com a chamada newfstatat() e incluem 4 outras chamadas do sistema. As chamadas de 3 close() e exit_group() são o tempo de execução de C fazendo limpeza após a função main() de rm retornar ou sair.

Para fazer um editorial um pouco, esse é o tipo de coisa que faz as pessoas desejarem por executáveis vinculados estaticamente e tentar várias coisas como sta.li ou Morpheus .